mysql explain参数解析

建表语句

-- ----------------------------

-- Records of departments

-- ----------------------------

INSERT INTO `departments` VALUES (10, 'Adm', 200, 1700);

INSERT INTO `departments` VALUES (20, 'Mar', 201, 1800);

INSERT INTO `departments` VALUES (30, 'Pur', 114, 1700);

INSERT INTO `departments` VALUES (40, 'Hum', 203, 2400);

INSERT INTO `departments` VALUES (50, 'Shi', 121, 1500);

INSERT INTO `departments` VALUES (60, 'IT', 103, 1400);

INSERT INTO `departments` VALUES (70, 'Pub', 204, 2700);

INSERT INTO `departments` VALUES (80, 'Sal', 145, 2500);

INSERT INTO `departments` VALUES (90, 'Exe', 100, 1700);

INSERT INTO `departments` VALUES (100, 'Fin', 108, 1700);

INSERT INTO `departments` VALUES (110, 'Acc', 205, 1700);

INSERT INTO `departments` VALUES (120, 'Tre', NULL, 1700);

INSERT INTO `departments` VALUES (130, 'Cor', NULL, 1700);

INSERT INTO `departments` VALUES (140, 'Con', NULL, 1700);

INSERT INTO `departments` VALUES (150, 'Sha', NULL, 1700);

INSERT INTO `departments` VALUES (160, 'Ben', NULL, 1700);

INSERT INTO `departments` VALUES (170, 'Man', NULL, 1700);

INSERT INTO `departments` VALUES (180, 'Con', NULL, 1700);

INSERT INTO `departments` VALUES (190, 'Con', NULL, 1700);

INSERT INTO `departments` VALUES (200, 'Ope', NULL, 1700);

INSERT INTO `departments` VALUES (210, 'IT ', NULL, 1700);

INSERT INTO `departments` VALUES (220, 'NOC', NULL, 1700);

INSERT INTO `departments` VALUES (230, 'IT ', NULL, 1700);

INSERT INTO `departments` VALUES (240, 'Gov', NULL, 1700);

INSERT INTO `departments` VALUES (250, 'Ret', NULL, 1700);

INSERT INTO `departments` VALUES (260, 'Rec', NULL, 1700);

INSERT INTO `departments` VALUES (270, 'Pay', NULL, 1700);

-- ----------------------------

-- Table structure for employees

-- ----------------------------

DROP TABLE IF EXISTS `employees`;

CREATE TABLE `employees`  (

  `employee_id` int(6) NOT NULL AUTO_INCREMENT,

  `first_name` varchar(20) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  `last_name` varchar(25) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  `email` varchar(25) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  `phone_number` varchar(20) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  `job_id` varchar(10) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  `salary` double(10, 2) NULL DEFAULT NULL,

  `commission_pct` double(4, 2) NULL DEFAULT NULL,

  `manager_id` int(6) NULL DEFAULT NULL,

  `department_id` int(4) NULL DEFAULT NULL,

  `hiredate` datetime NULL DEFAULT NULL,

  PRIMARY KEY (`employee_id`) USING BTREE,

  INDEX `dept_id_fk`(`department_id`) USING BTREE,

  INDEX `job_id_fk`(`job_id`) USING BTREE,

  CONSTRAINT `dept_id_fk` FOREIGN KEY (`department_id`) REFERENCES `departments` (`department_id`) ON DELETE RESTRICT ON UPDATE RESTRICT,

  CONSTRAINT `job_id_fk` FOREIGN KEY (`job_id`) REFERENCES `jobs` (`job_id`) ON DELETE RESTRICT ON UPDATE RESTRICT

) ENGINE = InnoDB AUTO_INCREMENT = 207 CHARACTER SET = gb2312 COLLATE = gb2312_chinese_ci ROW_FORMAT = Compact;

-- ----------------------------

-- Records of employees

-- ----------------------------

INSERT INTO `employees` VALUES (100, 'Steven', 'K_ing', 'SKING', '515.123.4567', 'AD_PRES', 24000.00, NULL, NULL, 90, '1992-04-03 00:00:00');

INSERT INTO `employees` VALUES (101, 'Neena', 'Kochhar', 'NKOCHHAR', '515.123.4568', 'AD_VP', 17000.00, NULL, 100, 90, '1992-04-03 00:00:00');

INSERT INTO `employees` VALUES (102, 'Lex', 'De Haan', 'LDEHAAN', '515.123.4569', 'AD_VP', 17000.00, NULL, 100, 90, '1992-04-03 00:00:00');

INSERT INTO `employees` VALUES (103, 'Alexander', 'Hunold', 'AHUNOLD', '590.423.4567', 'IT_PROG', 9000.00, NULL, 102, 60, '1992-04-03 00:00:00');

INSERT INTO `employees` VALUES (104, 'Bruce', 'Ernst', 'BERNST', '590.423.4568', 'IT_PROG', 6000.00, NULL, 103, 60, '1992-04-03 00:00:00');

INSERT INTO `employees` VALUES (105, 'David', 'Austin', 'DAUSTIN', '590.423.4569', 'IT_PROG', 4800.00, NULL, 103, 60, '1998-03-03 00:00:00');

INSERT INTO `employees` VALUES (106, 'Valli', 'Pataballa', 'VPATABAL', '590.423.4560', 'IT_PROG', 4800.00, NULL, 103, 60, '1998-03-03 00:00:00');

INSERT INTO `employees` VALUES (107, 'Diana', 'Lorentz', 'DLORENTZ', '590.423.5567', 'IT_PROG', 4200.00, NULL, 103, 60, '1998-03-03 00:00:00');

INSERT INTO `employees` VALUES (108, 'Nancy', 'Greenberg', 'NGREENBE', '515.124.4569', 'FI_MGR', 12000.00, NULL, 101, 100, '1998-03-03 00:00:00');

INSERT INTO `employees` VALUES (109, 'Daniel', 'Faviet', 'DFAVIET', '515.124.4169', 'FI_ACCOUNT', 9000.00, NULL, 108, 100, '1998-03-03 00:00:00');

INSERT INTO `employees` VALUES (110, 'John', 'Chen', 'JCHEN', '515.124.4269', 'FI_ACCOUNT', 8200.00, NULL, 108, 100, '2000-09-09 00:00:00');

INSERT INTO `employees` VALUES (111, 'Ismael', 'Sciarra', 'ISCIARRA', '515.124.4369', 'FI_ACCOUNT', 7700.00, NULL, 108, 100, '2000-09-09 00:00:00');

INSERT INTO `employees` VALUES (112, 'Jose Manuel', 'Urman', 'JMURMAN', '515.124.4469', 'FI_ACCOUNT', 7800.00, NULL, 108, 100, '2000-09-09 00:00:00');

INSERT INTO `employees` VALUES (113, 'Luis', 'Popp', 'LPOPP', '515.124.4567', 'FI_ACCOUNT', 6900.00, NULL, 108, 100, '2000-09-09 00:00:00');

INSERT INTO `employees` VALUES (114, 'Den', 'Raphaely', 'DRAPHEAL', '515.127.4561', 'PU_MAN', 11000.00, NULL, 100, 30, '2000-09-09 00:00:00');

INSERT INTO `employees` VALUES (115, 'Alexander', 'Khoo', 'AKHOO', '515.127.4562', 'PU_CLERK', 3100.00, NULL, 114, 30, '2000-09-09 00:00:00');

INSERT INTO `employees` VALUES (116, 'Shelli', 'Baida', 'SBAIDA', '515.127.4563', 'PU_CLERK', 2900.00, NULL, 114, 30, '2000-09-09 00:00:00');

INSERT INTO `employees` VALUES (117, 'Sigal', 'Tobias', 'STOBIAS', '515.127.4564', 'PU_CLERK', 2800.00, NULL, 114, 30, '2000-09-09 00:00:00');

INSERT INTO `employees` VALUES (118, 'Guy', 'Himuro', 'GHIMURO', '515.127.4565', 'PU_CLERK', 2600.00, NULL, 114, 30, '2000-09-09 00:00:00');

INSERT INTO `employees` VALUES (119, 'Karen', 'Colmenares', 'KCOLMENA', '515.127.4566', 'PU_CLERK', 2500.00, NULL, 114, 30, '2000-09-09 00:00:00');

INSERT INTO `employees` VALUES (120, 'Matthew', 'Weiss', 'MWEISS', '650.123.1234', 'ST_MAN', 8000.00, NULL, 100, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (121, 'Adam', 'Fripp', 'AFRIPP', '650.123.2234', 'ST_MAN', 8200.00, NULL, 100, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (122, 'Payam', 'Kaufling', 'PKAUFLIN', '650.123.3234', 'ST_MAN', 7900.00, NULL, 100, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (123, 'Shanta', 'Vollman', 'SVOLLMAN', '650.123.4234', 'ST_MAN', 6500.00, NULL, 100, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (124, 'Kevin', 'Mourgos', 'KMOURGOS', '650.123.5234', 'ST_MAN', 5800.00, NULL, 100, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (125, 'Julia', 'Nayer', 'JNAYER', '650.124.1214', 'ST_CLERK', 3200.00, NULL, 120, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (126, 'Irene', 'Mikkilineni', 'IMIKKILI', '650.124.1224', 'ST_CLERK', 2700.00, NULL, 120, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (127, 'James', 'Landry', 'JLANDRY', '650.124.1334', 'ST_CLERK', 2400.00, NULL, 120, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (128, 'Steven', 'Markle', 'SMARKLE', '650.124.1434', 'ST_CLERK', 2200.00, NULL, 120, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (129, 'Laura', 'Bissot', 'LBISSOT', '650.124.5234', 'ST_CLERK', 3300.00, NULL, 121, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (130, 'Mozhe', 'Atkinson', 'MATKINSO', '650.124.6234', 'ST_CLERK', 2800.00, NULL, 121, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (131, 'James', 'Marlow', 'JAMRLOW', '650.124.7234', 'ST_CLERK', 2500.00, NULL, 121, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (132, 'TJ', 'Olson', 'TJOLSON', '650.124.8234', 'ST_CLERK', 2100.00, NULL, 121, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (133, 'Jason', 'Mallin', 'JMALLIN', '650.127.1934', 'ST_CLERK', 3300.00, NULL, 122, 50, '2004-02-06 00:00:00');

INSERT INTO `employees` VALUES (134, 'Michael', 'Rogers', 'MROGERS', '650.127.1834', 'ST_CLERK', 2900.00, NULL, 122, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (135, 'Ki', 'Gee', 'KGEE', '650.127.1734', 'ST_CLERK', 2400.00, NULL, 122, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (136, 'Hazel', 'Philtanker', 'HPHILTAN', '650.127.1634', 'ST_CLERK', 2200.00, NULL, 122, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (137, 'Renske', 'Ladwig', 'RLADWIG', '650.121.1234', 'ST_CLERK', 3600.00, NULL, 123, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (138, 'Stephen', 'Stiles', 'SSTILES', '650.121.2034', 'ST_CLERK', 3200.00, NULL, 123, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (139, 'John', 'Seo', 'JSEO', '650.121.2019', 'ST_CLERK', 2700.00, NULL, 123, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (140, 'Joshua', 'Patel', 'JPATEL', '650.121.1834', 'ST_CLERK', 2500.00, NULL, 123, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (141, 'Trenna', 'Rajs', 'TRAJS', '650.121.8009', 'ST_CLERK', 3500.00, NULL, 124, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (142, 'Curtis', 'Davies', 'CDAVIES', '650.121.2994', 'ST_CLERK', 3100.00, NULL, 124, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (143, 'Randall', 'Matos', 'RMATOS', '650.121.2874', 'ST_CLERK', 2600.00, NULL, 124, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (144, 'Peter', 'Vargas', 'PVARGAS', '650.121.2004', 'ST_CLERK', 2500.00, NULL, 124, 50, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (145, 'John', 'Russell', 'JRUSSEL', '011.44.1344.429268', 'SA_MAN', 14000.00, 0.40, 100, 80, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (146, 'Karen', 'Partners', 'KPARTNER', '011.44.1344.467268', 'SA_MAN', 13500.00, 0.30, 100, 80, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (147, 'Alberto', 'Errazuriz', 'AERRAZUR', '011.44.1344.429278', 'SA_MAN', 12000.00, 0.30, 100, 80, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (148, 'Gerald', 'Cambrault', 'GCAMBRAU', '011.44.1344.619268', 'SA_MAN', 11000.00, 0.30, 100, 80, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (149, 'Eleni', 'Zlotkey', 'EZLOTKEY', '011.44.1344.429018', 'SA_MAN', 10500.00, 0.20, 100, 80, '2002-12-23 00:00:00');

INSERT INTO `employees` VALUES (150, 'Peter', 'Tucker', 'PTUCKER', '011.44.1344.129268', 'SA_REP', 10000.00, 0.30, 145, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (151, 'David', 'Bernstein', 'DBERNSTE', '011.44.1344.345268', 'SA_REP', 9500.00, 0.25, 145, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (152, 'Peter', 'Hall', 'PHALL', '011.44.1344.478968', 'SA_REP', 9000.00, 0.25, 145, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (153, 'Christopher', 'Olsen', 'COLSEN', '011.44.1344.498718', 'SA_REP', 8000.00, 0.20, 145, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (154, 'Nanette', 'Cambrault', 'NCAMBRAU', '011.44.1344.987668', 'SA_REP', 7500.00, 0.20, 145, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (155, 'Oliver', 'Tuvault', 'OTUVAULT', '011.44.1344.486508', 'SA_REP', 7000.00, 0.15, 145, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (156, 'Janette', 'K_ing', 'JKING', '011.44.1345.429268', 'SA_REP', 10000.00, 0.35, 146, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (157, 'Patrick', 'Sully', 'PSULLY', '011.44.1345.929268', 'SA_REP', 9500.00, 0.35, 146, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (158, 'Allan', 'McEwen', 'AMCEWEN', '011.44.1345.829268', 'SA_REP', 9000.00, 0.35, 146, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (159, 'Lindsey', 'Smith', 'LSMITH', '011.44.1345.729268', 'SA_REP', 8000.00, 0.30, 146, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (160, 'Louise', 'Doran', 'LDORAN', '011.44.1345.629268', 'SA_REP', 7500.00, 0.30, 146, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (161, 'Sarath', 'Sewall', 'SSEWALL', '011.44.1345.529268', 'SA_REP', 7000.00, 0.25, 146, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (162, 'Clara', 'Vishney', 'CVISHNEY', '011.44.1346.129268', 'SA_REP', 10500.00, 0.25, 147, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (163, 'Danielle', 'Greene', 'DGREENE', '011.44.1346.229268', 'SA_REP', 9500.00, 0.15, 147, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (164, 'Mattea', 'Marvins', 'MMARVINS', '011.44.1346.329268', 'SA_REP', 7200.00, 0.10, 147, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (165, 'David', 'Lee', 'DLEE', '011.44.1346.529268', 'SA_REP', 6800.00, 0.10, 147, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (166, 'Sundar', 'Ande', 'SANDE', '011.44.1346.629268', 'SA_REP', 6400.00, 0.10, 147, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (167, 'Amit', 'Banda', 'ABANDA', '011.44.1346.729268', 'SA_REP', 6200.00, 0.10, 147, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (168, 'Lisa', 'Ozer', 'LOZER', '011.44.1343.929268', 'SA_REP', 11500.00, 0.25, 148, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (169, 'Harrison', 'Bloom', 'HBLOOM', '011.44.1343.829268', 'SA_REP', 10000.00, 0.20, 148, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (170, 'Tayler', 'Fox', 'TFOX', '011.44.1343.729268', 'SA_REP', 9600.00, 0.20, 148, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (171, 'William', 'Smith', 'WSMITH', '011.44.1343.629268', 'SA_REP', 7400.00, 0.15, 148, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (172, 'Elizabeth', 'Bates', 'EBATES', '011.44.1343.529268', 'SA_REP', 7300.00, 0.15, 148, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (173, 'Sundita', 'Kumar', 'SKUMAR', '011.44.1343.329268', 'SA_REP', 6100.00, 0.10, 148, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (174, 'Ellen', 'Abel', 'EABEL', '011.44.1644.429267', 'SA_REP', 11000.00, 0.30, 149, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (175, 'Alyssa', 'Hutton', 'AHUTTON', '011.44.1644.429266', 'SA_REP', 8800.00, 0.25, 149, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (176, 'Jonathon', 'Taylor', 'JTAYLOR', '011.44.1644.429265', 'SA_REP', 8600.00, 0.20, 149, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (177, 'Jack', 'Livingston', 'JLIVINGS', '011.44.1644.429264', 'SA_REP', 8400.00, 0.20, 149, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (178, 'Kimberely', 'Grant', 'KGRANT', '011.44.1644.429263', 'SA_REP', 7000.00, 0.15, 149, NULL, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (179, 'Charles', 'Johnson', 'CJOHNSON', '011.44.1644.429262', 'SA_REP', 6200.00, 0.10, 149, 80, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (180, 'Winston', 'Taylor', 'WTAYLOR', '650.507.9876', 'SH_CLERK', 3200.00, NULL, 120, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (181, 'Jean', 'Fleaur', 'JFLEAUR', '650.507.9877', 'SH_CLERK', 3100.00, NULL, 120, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (182, 'Martha', 'Sullivan', 'MSULLIVA', '650.507.9878', 'SH_CLERK', 2500.00, NULL, 120, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (183, 'Girard', 'Geoni', 'GGEONI', '650.507.9879', 'SH_CLERK', 2800.00, NULL, 120, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (184, 'Nandita', 'Sarchand', 'NSARCHAN', '650.509.1876', 'SH_CLERK', 4200.00, NULL, 121, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (185, 'Alexis', 'Bull', 'ABULL', '650.509.2876', 'SH_CLERK', 4100.00, NULL, 121, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (186, 'Julia', 'Dellinger', 'JDELLING', '650.509.3876', 'SH_CLERK', 3400.00, NULL, 121, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (187, 'Anthony', 'Cabrio', 'ACABRIO', '650.509.4876', 'SH_CLERK', 3000.00, NULL, 121, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (188, 'Kelly', 'Chung', 'KCHUNG', '650.505.1876', 'SH_CLERK', 3800.00, NULL, 122, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (189, 'Jennifer', 'Dilly', 'JDILLY', '650.505.2876', 'SH_CLERK', 3600.00, NULL, 122, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (190, 'Timothy', 'Gates', 'TGATES', '650.505.3876', 'SH_CLERK', 2900.00, NULL, 122, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (191, 'Randall', 'Perkins', 'RPERKINS', '650.505.4876', 'SH_CLERK', 2500.00, NULL, 122, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (192, 'Sarah', 'Bell', 'SBELL', '650.501.1876', 'SH_CLERK', 4000.00, NULL, 123, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (193, 'Britney', 'Everett', 'BEVERETT', '650.501.2876', 'SH_CLERK', 3900.00, NULL, 123, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (194, 'Samuel', 'McCain', 'SMCCAIN', '650.501.3876', 'SH_CLERK', 3200.00, NULL, 123, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (195, 'Vance', 'Jones', 'VJONES', '650.501.4876', 'SH_CLERK', 2800.00, NULL, 123, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (196, 'Alana', 'Walsh', 'AWALSH', '650.507.9811', 'SH_CLERK', 3100.00, NULL, 124, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (197, 'Kevin', 'Feeney', 'KFEENEY', '650.507.9822', 'SH_CLERK', 3000.00, NULL, 124, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (198, 'Donald', 'OConnell', 'DOCONNEL', '650.507.9833', 'SH_CLERK', 2600.00, NULL, 124, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (199, 'Douglas', 'Grant', 'DGRANT', '650.507.9844', 'SH_CLERK', 2600.00, NULL, 124, 50, '2014-03-05 00:00:00');

INSERT INTO `employees` VALUES (200, 'Jennifer', 'Whalen', 'JWHALEN', '515.123.4444', 'AD_ASST', 4400.00, NULL, 101, 10, '2016-03-03 00:00:00');

INSERT INTO `employees` VALUES (201, 'Michael', 'Hartstein', 'MHARTSTE', '515.123.5555', 'MK_MAN', 13000.00, NULL, 100, 20, '2016-03-03 00:00:00');

INSERT INTO `employees` VALUES (202, 'Pat', 'Fay', 'PFAY', '603.123.6666', 'MK_REP', 6000.00, NULL, 201, 20, '2016-03-03 00:00:00');

INSERT INTO `employees` VALUES (203, 'Susan', 'Mavris', 'SMAVRIS', '515.123.7777', 'HR_REP', 6500.00, NULL, 101, 40, '2016-03-03 00:00:00');

INSERT INTO `employees` VALUES (204, 'Hermann', 'Baer', 'HBAER', '515.123.8888', 'PR_REP', 10000.00, NULL, 101, 70, '2016-03-03 00:00:00');

INSERT INTO `employees` VALUES (205, 'Shelley', 'Higgins', 'SHIGGINS', '515.123.8080', 'AC_MGR', 12000.00, NULL, 101, 110, '2016-03-03 00:00:00');

INSERT INTO `employees` VALUES (206, 'William', 'Gietz', 'WGIETZ', '515.123.8181', 'AC_ACCOUNT', 8300.00, NULL, 205, 110, '2016-03-03 00:00:00');

-- ----------------------------

-- Table structure for job_grades

-- ----------------------------

DROP TABLE IF EXISTS `job_grades`;

CREATE TABLE `job_grades`  (

  `grade_level` varchar(3) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,

  `lowest_sal` int(11) NULL DEFAULT NULL,

  `highest_sal` int(11) NULL DEFAULT NULL

) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

-- ----------------------------

-- Records of job_grades

-- ----------------------------

INSERT INTO `job_grades` VALUES ('A', 1000, 2999);

INSERT INTO `job_grades` VALUES ('B', 3000, 5999);

INSERT INTO `job_grades` VALUES ('C', 6000, 9999);

INSERT INTO `job_grades` VALUES ('D', 10000, 14999);

INSERT INTO `job_grades` VALUES ('E', 15000, 24999);

INSERT INTO `job_grades` VALUES ('F', 25000, 40000);

-- ----------------------------

-- Table structure for jobs

-- ----------------------------

DROP TABLE IF EXISTS `jobs`;

CREATE TABLE `jobs`  (

  `job_id` varchar(10) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NOT NULL,

  `job_title` varchar(35) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  `min_salary` int(6) NULL DEFAULT NULL,

  `max_salary` int(6) NULL DEFAULT NULL,

  PRIMARY KEY (`job_id`) USING BTREE

) ENGINE = InnoDB CHARACTER SET = gb2312 COLLATE = gb2312_chinese_ci ROW_FORMAT = Compact;

-- ----------------------------

-- Records of jobs

-- ----------------------------

INSERT INTO `jobs` VALUES ('AC_ACCOUNT', 'Public Accountant', 4200, 9000);

INSERT INTO `jobs` VALUES ('AC_MGR', 'Accounting Manager', 8200, 16000);

INSERT INTO `jobs` VALUES ('AD_ASST', 'Administration Assistant', 3000, 6000);

INSERT INTO `jobs` VALUES ('AD_PRES', 'President', 20000, 40000);

INSERT INTO `jobs` VALUES ('AD_VP', 'Administration Vice President', 15000, 30000);

INSERT INTO `jobs` VALUES ('FI_ACCOUNT', 'Accountant', 4200, 9000);

INSERT INTO `jobs` VALUES ('FI_MGR', 'Finance Manager', 8200, 16000);

INSERT INTO `jobs` VALUES ('HR_REP', 'Human Resources Representative', 4000, 9000);

INSERT INTO `jobs` VALUES ('IT_PROG', 'Programmer', 4000, 10000);

INSERT INTO `jobs` VALUES ('MK_MAN', 'Marketing Manager', 9000, 15000);

INSERT INTO `jobs` VALUES ('MK_REP', 'Marketing Representative', 4000, 9000);

INSERT INTO `jobs` VALUES ('PR_REP', 'Public Relations Representative', 4500, 10500);

INSERT INTO `jobs` VALUES ('PU_CLERK', 'Purchasing Clerk', 2500, 5500);

INSERT INTO `jobs` VALUES ('PU_MAN', 'Purchasing Manager', 8000, 15000);

INSERT INTO `jobs` VALUES ('SA_MAN', 'Sales Manager', 10000, 20000);

INSERT INTO `jobs` VALUES ('SA_REP', 'Sales Representative', 6000, 12000);

INSERT INTO `jobs` VALUES ('SH_CLERK', 'Shipping Clerk', 2500, 5500);

INSERT INTO `jobs` VALUES ('ST_CLERK', 'Stock Clerk', 2000, 5000);

INSERT INTO `jobs` VALUES ('ST_MAN', 'Stock Manager', 5500, 8500);

-- ----------------------------

-- Table structure for locations

-- ----------------------------

DROP TABLE IF EXISTS `locations`;

CREATE TABLE `locations`  (

  `location_id` int(11) NOT NULL AUTO_INCREMENT,

  `street_address` varchar(40) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  `postal_code` varchar(12) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  `city` varchar(30) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  `state_province` varchar(25) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  `country_id` varchar(2) CHARACTER SET gb2312 COLLATE gb2312_chinese_ci NULL DEFAULT NULL,

  PRIMARY KEY (`location_id`) USING BTREE

) ENGINE = InnoDB AUTO_INCREMENT = 3201 CHARACTER SET = gb2312 COLLATE = gb2312_chinese_ci ROW_FORMAT = Compact;

-- ----------------------------

-- Records of locations

-- ----------------------------

INSERT INTO `locations` VALUES (1000, '1297 Via Cola di Rie', '00989', 'Roma', NULL, 'IT');

INSERT INTO `locations` VALUES (1100, '93091 Calle della Testa', '10934', 'Venice', NULL, 'IT');

INSERT INTO `locations` VALUES (1200, '2017 Shinjuku-ku', '1689', 'Tokyo', 'Tokyo Prefecture', 'JP');

INSERT INTO `locations` VALUES (1300, '9450 Kamiya-cho', '6823', 'Hiroshima', NULL, 'JP');

INSERT INTO `locations` VALUES (1400, '2014 Jabberwocky Rd', '26192', 'Southlake', 'Texas', 'US');

INSERT INTO `locations` VALUES (1500, '2011 Interiors Blvd', '99236', 'South San Francisco', 'California', 'US');

INSERT INTO `locations` VALUES (1600, '2007 Zagora St', '50090', 'South Brunswick', 'New Jersey', 'US');

INSERT INTO `locations` VALUES (1700, '2004 Charade Rd', '98199', 'Seattle', 'Washington', 'US');

INSERT INTO `locations` VALUES (1800, '147 Spadina Ave', 'M5V 2L7', 'Toronto', 'Ontario', 'CA');

INSERT INTO `locations` VALUES (1900, '6092 Boxwood St', 'YSW 9T2', 'Whitehorse', 'Yukon', 'CA');

INSERT INTO `locations` VALUES (2000, '40-5-12 Laogianggen', '190518', 'Beijing', NULL, 'CN');

INSERT INTO `locations` VALUES (2100, '1298 Vileparle (E)', '490231', 'Bombay', 'Maharashtra', 'IN');

INSERT INTO `locations` VALUES (2200, '12-98 Victoria Street', '2901', 'Sydney', 'New South Wales', 'AU');

INSERT INTO `locations` VALUES (2300, '198 Clementi North', '540198', 'Singapore', NULL, 'SG');

INSERT INTO `locations` VALUES (2400, '8204 Arthur St', NULL, 'London', NULL, 'UK');

INSERT INTO `locations` VALUES (2500, 'Magdalen Centre, The Oxford Science Park', 'OX9 9ZB', 'Oxford', 'Oxford', 'UK');

INSERT INTO `locations` VALUES (2600, '9702 Chester Road', '09629850293', 'Stretford', 'Manchester', 'UK');

INSERT INTO `locations` VALUES (2700, 'Schwanthalerstr. 7031', '80925', 'Munich', 'Bavaria', 'DE');

INSERT INTO `locations` VALUES (2800, 'Rua Frei Caneca 1360 ', '01307-002', 'Sao Paulo', 'Sao Paulo', 'BR');

INSERT INTO `locations` VALUES (2900, '20 Rue des Corps-Saints', '1730', 'Geneva', 'Geneve', 'CH');

INSERT INTO `locations` VALUES (3000, 'Murtenstrasse 921', '3095', 'Bern', 'BE', 'CH');

INSERT INTO `locations` VALUES (3100, 'Pieter Breughelstraat 837', '3029SK', 'Utrecht', 'Utrecht', 'NL');

INSERT INTO `locations` VALUES (3200, 'Mariano Escobedo 9991', '11932', 'Mexico City', 'Distrito Federal,', 'MX');

-- 2

SET NAMES utf8mb4;

SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------

-- Table structure for admin

-- ----------------------------

DROP TABLE IF EXISTS `admin`;

CREATE TABLE `admin`  (

  `id` int(11) NOT NULL AUTO_INCREMENT,

  `username` varchar(10) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

  `password` varchar(10) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

  PRIMARY KEY (`id`) USING BTREE

) ENGINE = InnoDB AUTO_INCREMENT = 3 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

-- ----------------------------

-- Records of admin

-- ----------------------------

INSERT INTO `admin` VALUES (1, 'john', '8888');

INSERT INTO `admin` VALUES (2, 'lyt', '6666');

-- ----------------------------

-- Table structure for beauty

-- ----------------------------

DROP TABLE IF EXISTS `beauty`;

CREATE TABLE `beauty`  (

  `id` int(11) NOT NULL AUTO_INCREMENT,

  `name` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

  `sex` char(1) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT 'Ů',

  `borndate` datetime NULL DEFAULT '1987-01-01 00:00:00',

  `phone` varchar(11) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

  `photo` blob NULL,

  `boyfriend_id` int(11) NULL DEFAULT NULL,

  PRIMARY KEY (`id`) USING BTREE

) ENGINE = InnoDB AUTO_INCREMENT = 13 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

-- ----------------------------

-- Records of beauty

-- ----------------------------

INSERT INTO `beauty` VALUES (1, '柳岩', '女', '1988-02-03 00:00:00', '18209876577', NULL, 8);

INSERT INTO `beauty` VALUES (2, '苍老师', '女', '1987-12-30 00:00:00', '18219876577', NULL, 9);

INSERT INTO `beauty` VALUES (3, 'Angelababy', '女', '1989-02-03 00:00:00', '18209876567', NULL, 3);

INSERT INTO `beauty` VALUES (4, '热巴', '女', '1993-02-03 00:00:00', '18209876579', NULL, 2);

INSERT INTO `beauty` VALUES (5, '周冬雨', '女', '1992-02-03 00:00:00', '18209179577', NULL, 9);

INSERT INTO `beauty` VALUES (6, '周芷若', '女', '1988-02-03 00:00:00', '18209876577', NULL, 1);

INSERT INTO `beauty` VALUES (7, '岳灵珊', '女', '1987-12-30 00:00:00', '18219876577', NULL, 9);

INSERT INTO `beauty` VALUES (8, '小昭', '女', '1989-02-03 00:00:00', '18209876567', NULL, 1);

INSERT INTO `beauty` VALUES (9, '双儿', '女', '1993-02-03 00:00:00', '18209876579', NULL, 9);

INSERT INTO `beauty` VALUES (10, '王语嫣', '女', '1992-02-03 00:00:00', '18209179577', NULL, 4);

INSERT INTO `beauty` VALUES (11, '夏雪', '女', '1993-02-03 00:00:00', '18209876579', NULL, 9);

INSERT INTO `beauty` VALUES (12, '赵敏', '女', '1992-02-03 00:00:00', '18209179577', NULL, 1);

-- ----------------------------

-- Table structure for boys

-- ----------------------------

DROP TABLE IF EXISTS `boys`;

CREATE TABLE `boys`  (

  `id` int(11) NOT NULL AUTO_INCREMENT,

  `boyName` varchar(20) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,

  `userCP` int(11) NULL DEFAULT NULL,

  PRIMARY KEY (`id`) USING BTREE

) ENGINE = InnoDB AUTO_INCREMENT = 5 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Compact;

-- ----------------------------

-- Records of boys

-- ----------------------------

INSERT INTO `boys` VALUES (1, '张无忌', 100);

INSERT INTO `boys` VALUES (2, '鹿晗', 800);

INSERT INTO `boys` VALUES (3, '黄晓', 50);

INSERT INTO `boys` VALUES (4, '段誉', 300);

SET FOREIGN_KEY_CHECKS = 1;

-- 3

USE myemployees;

CREATE TABLE job_grades(

  grade_level VARCHAR(3),

  lowest_sal  INT,

  highest_sal INT

);

INSERT INTO job_grades

VALUES ('A', 1000, 2999);

INSERT INTO job_grades

VALUES ('B', 3000, 5999);

INSERT INTO job_grades

VALUES('C', 6000, 9999);

INSERT INTO job_grades

VALUES('D', 10000, 14999);

INSERT INTO job_grades

VALUES('E', 15000, 24999);

INSERT INTO job_grades

VALUES('F', 25000, 40000);

对写的sql前添加explain可以查看该sql语句的执行计划

mysql> explain select first_name, last_name, email, job_id from employees WHERE salary BETWEEN 9000 and 17000 ORDER BY salary desc;

+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+

| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                       |

+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+

|  1 | SIMPLE      | employees | NULL       | ALL  | NULL          | NULL | NULL    | NULL |  107 |    11.11 | Using where; Using filesort |

+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+

1 row in set, 1 warning (0.00 sec)

如上所示会显示以下参数:

包含一组数组, 表示查询中执行select子句或操作表的顺序
id 相同, 执行顺序由上至下, 否则id值越大(通常子查询会产生), 优先级越高, 越先被执行
id 列为 null 的表就是一个结果集, 不需要使用它来进行查询

demo:

EXPLAIN

SELECT

employees.employee_id,

employees.first_name,

employees.last_name,

employees.salary,

employees.job_id,

employees.phone_number,

employees.manager_id,

departments.department_name,

departments.location_id,

departments.manager_id,

jobs.min_salary,

jobs.job_title,

jobs.max_salary

FROM

departments

INNER JOIN employees ON employees.department_id = departments.department_id

INNER JOIN jobs ON employees.job_id = jobs.job_id

WHERE

employees.first_name in (select employees.first_name from employees)

显示结果:

+----+--------------+-------------+------------+--------+----------------------+------------+---------+------------------------------+------+----------+-------------+

| id | select_type  | table       | partitions | type   | possible_keys        | key        | key_len | ref                          | rows | filtered | Extra       |

+----+--------------+-------------+------------+--------+----------------------+------------+---------+------------------------------+------+----------+-------------+

|  1 | SIMPLE       | jobs        | NULL       | ALL    | PRIMARY              | NULL       | NULL    | NULL                         |   19 |   100.00 | NULL        |

|  1 | SIMPLE       | employees   | NULL       | ref    | dept_id_fk,job_id_fk | job_id_fk  | 23      | test.jobs.job_id             |    5 |   100.00 | Using where |

|  1 | SIMPLE       | departments | NULL       | eq_ref | PRIMARY              | PRIMARY    | 4       | test.employees.department_id |    1 |   100.00 | NULL        |

|  1 | SIMPLE       | <subquery2> | NULL       | eq_ref | <auto_key>           | <auto_key> | 43      | test.employees.first_name    |    1 |   100.00 | NULL        |

|  2 | MATERIALIZED | employees   | NULL       | ALL    | NULL                 | NULL       | NULL    | NULL                         |  107 |   100.00 | NULL        |

+----+--------------+-------------+------------+--------+----------------------+------------+---------+------------------------------+------+----------+-------------+

5 rows in set, 1 warning (0.00 sec)

select_type

simple
- 简单的select语句 (不包括union操作或子查询操作)

primary / union

Demo

EXPLAIN

SELECT

	first_name,

	last_name

FROM

	employees

WHERE

	salary = (

		SELECT

			MAX(salary)

		FROM

			employees

	);

执行结果:

+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+

| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |

+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+

|  1 | PRIMARY     | employees | NULL       | ALL  | NULL          | NULL | NULL    | NULL |  107 |    10.00 | Using where |

|  2 | SUBQUERY    | employees | NULL       | ALL  | NULL          | NULL | NULL    | NULL |  107 |   100.00 | NULL        |

+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+

2 rows in set, 1 warning (0.00 sec)

primary: 查询中最外层的SELECT (如两表做UNION或者存在子查询的外层的表操作为PRIMARY，内层的操作为UNION)
union: UNION操作中，查询中处于内层的SELECT（内层的SELECT语句与外层的SELECT语句没有依赖关系）

dependent union / union result

Demo

EXPLAIN

SELECT

	*

FROM

	employees

WHERE

	manager_id = 114

UNION

	SELECT

		*

	FROM

		employees

	WHERE

		job_id = "SH_CLERK";

执行结果:

+----+--------------+------------+------------+------+---------------+-----------+---------+-------+------+----------+-----------------+

| id | select_type  | table      | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra           |

+----+--------------+------------+------------+------+---------------+-----------+---------+-------+------+----------+-----------------+

|  1 | PRIMARY      | employees  | NULL       | ALL  | NULL          | NULL      | NULL    | NULL  |  107 |    10.00 | Using where     |

|  2 | UNION        | employees  | NULL       | ref  | job_id_fk     | job_id_fk | 23      | const |   20 |   100.00 | NULL            |

| NULL | UNION RESULT | <union1,2> | NULL       | ALL  | NULL          | NULL      | NULL    | NULL  | NULL |     NULL | Using temporary |

+----+--------------+------------+------------+------+---------------+-----------+---------+-------+------+----------+-----------------+

3 rows in set, 1 warning (0.00 sec)

department union: UNION操作中，查询中处于内层的SELECT（内层的SELECT语句与外层的SELECT语句有依赖关系）
union result: UNION操作的结果，id值通常为NULL

subquery / dependent subquery

subquery: 子查询中首个select
dependent subquery: 子查询中首个SELECT，但依赖于外层的表
- 需要注意的是dependent subquery会严重消耗性能。

Demo:

explain

SELECT

	first_name,

	last_name,

	phone_number

FROM

	employees e1

WHERE

	salary = (

		SELECT

			MAX(salary)

		FROM

			employees

		WHERE

			e1.employee_id = employees.employee_id

	);

执行结果:

+----+--------------------+-----------+------------+--------+---------------+---------+---------+---------------------+------+----------+-------------+

| id | select_type        | table     | partitions | type   | possible_keys | key     | key_len | ref                 | rows | filtered | Extra       |

+----+--------------------+-----------+------------+--------+---------------+---------+---------+---------------------+------+----------+-------------+

|  1 | PRIMARY            | e1        | NULL       | ALL    | NULL          | NULL    | NULL    | NULL                |  107 |   100.00 | Using where |

|  2 | DEPENDENT SUBQUERY | employees | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | test.e1.employee_id |    1 |   100.00 | NULL        |

+----+--------------------+-----------+------------+--------+---------------+---------+---------+---------------------+------+----------+-------------+

子查询的查询方式依赖于外面的查询结果以此例子解释:
- 先进行子查询外部的查询,得到一个结果集, 然后这个结果的每一行在跟select子查询的结果集进行匹配
- 外部结果集的每一行都要关联内部结果集一次

derived / materialized

derived: 被驱动的select子查询(子查询位于FROM子句)

Demo:

explain

SELECT

	e1.employee_id,

	e1.first_name,

	e1.last_name,

	e1.email

FROM

	employees e1

JOIN (

	SELECT

		employee_id,

		first_name,

		last_name,

		email

	FROM

		employees

	GROUP BY

		employee_id

) AS s1 ON e1.employee_id = s1.employee_id

AND e1.email = s1.email;

执行结果:

+----+-------------+------------+------------+--------+------------------------------+---------+---------+----------------+------+----------+-------------+

| id | select_type | table      | partitions | type   | possible_keys                | key     | key_len | ref            | rows | filtered | Extra       |

+----+-------------+------------+------------+--------+------------------------------+---------+---------+----------------+------+----------+-------------+

|  1 | PRIMARY     | <derived2> | NULL       | ALL    | NULL                         | NULL    | NULL    | NULL           |  107 |   100.00 | NULL        |

|  1 | PRIMARY     | e1         | NULL       | eq_ref | PRIMARY                      | PRIMARY | 4       | s1.employee_id |    1 |    10.00 | Using where |

|  2 | DERIVED     | employees  | NULL       | index  | PRIMARY,dept_id_fk,job_id_fk | PRIMARY | 4       | NULL           |  107 |   100.00 | NULL        |

+----+-------------+------------+------------+--------+------------------------------+---------+---------+----------------+------+----------+-------------+

3 rows in set, 1 warning (0.00 sec)

materialized：被物化的子查询

uncacheable subquery / uncacheable union
- uncacheable subquery: 对于外层的主表, 子查询不可被物化, 每次需要计算(耗时操作)
- uncacheable union: union操作中, 内层的不可被物化的子查询 (类似于uncacheable subquery)

table
- 显示这一步所访问数据库中表名称 (显示这一行的数据是关于哪张表的)，有时不是真实的表名字，可能是简称。
partitions
- 分区
type
- 对表访问方式，表示MySQL在表中找到所需行的方式，又称“访问类型”。
- 常用的类型有： ALL、index、range、 ref、eq_ref、const、system、NULL（从左到右，性能从差到好）
- ALL: 将执行全表查询(100万以上数据量需要优化, 加索引)
- Index: 索引, 只遍历索引树
- range: 只检索给定范围的行, 使用一个索引来选择行。
- ref(reference): 表示上述表的连接匹配条件, 即哪些列或常量被用于查找索引列上的值
- eq_ref: 类似ref，区别就在使用的索引是唯一索引，对于每个索引键值，表中只有一条记录匹配，简单来说，就是多表连接中使用primary key或者 unique key作为关联条件
- const、system: 当MySQL对查询某部分进行优化，并转换为一个常量时，使用这些类型访问。如将主键置于where列表中，MySQL就能将该查询转换为一个常量，system是const类型的特例，当查询的表只有一行的情况下，使用system 。
- NULL: MySQL在优化过程中分解语句，执行时甚至不用访问表或索引，例如从一个索引列里选取最小值可以通过单独索引查找完成。
possible_keys
- 指出MySQL能使用哪个索引在表中找到记录, 查询涉及到的字段上若存在索引, 则该索引将被列出, 但不一定被查询使用。
key
- key列显示MySQL实际决定使用的键(索引), 必然包含在possible_keys中
- 如果没有选择索引，键是NULL。要想强制MySQL使用或忽视possible_keys列中的索引，在查询中使用FORCE INDEX、USE INDEX或者IGNORE INDEX。
key_len
- 表示索引中使用的字节数, 可通过对该列计算查询中使用的索引的长度
- key_len显示的值为索引字段的最大可能长度，并非实际使用长度，即key_len是根据表定义计算而得，不是通过表内检索出的
- 不损失精确性的情况下，长度越短越好
ref
- 列与索引的比较，表示上述表的连接匹配条件，即哪些列或常量被用于查找索引列上的值
rows
- 估算出结果集行数，表示MySQL根据表统计信息及索引选用情况，估算的找到所需的记录所需要读取的行数
filtered
- 是一个半分比的值，rows * filtered/100 可以估算出将要和 explain 中前一个表进行连接的行数
Extra
- Using filesort
  - 说明mysql会对数据使用一个外部的索引排序, 而不是按照表内的索引顺序进行读取。
  - MySQL中无法利用索引完成的排序操作称为"文件排序"
  - 数据量大时, 比较危险(九死一生)
- Using temporary
  - 使用了临时表保存中间结果, MySQL在对查询结果排序时使用临时表。常见于排序order by 和分组查询 group by。
  - 数据量大时, 非常危险(十死无生)
- Using index
  - 表示相应的select操作中使用了覆盖索引(Covering Index), 避免访问了表的数据行, 效率不错！
  - 如果同时出现using where, 表明索引被来执行索引KV的查找
  - 如果没有同时出现using where, 表明索引用来读取数据而非执行查找动作
- Covering Index
  - select 的数据列只用从索引中就能够取得, 不必读取数据行, MySQL可以利用索引返回select列表中的字段, 而不必根据索引在此读取数据文件, 换句话说查询列要被所建的索引覆盖
  - 注意:
    - 如果要使用覆盖索引, 一定要注意select列表中只取出需要的列, 不可select*;
    - 如果将所有字段一起做索引会导致文件过大, 查询性能下降。
- Using where: 使用了where过滤
- Using join buffer: 使用了连接缓存
- impossible where: where子句的值总是false, 不能用来获取任何元组
- select tables optimized away: 在没有GroupBy子句的情况下, 基于索引优化MIN/MAX操作
- distinct: 优化distinct操作, 在找到第一匹配的元组后即停止找同样值的动作

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mysql explain参数解析

建表语句

对写的sql前添加explain可以查看该sql语句的执行计划

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