User-Defined Variables

 mysql> SET @w := SELECT COUNT(*) FROM course WHERE cteacher='程军';
 ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version
  the right syntax to use near 'SELECT COUNT(*) FROM course WHERE cteacher='程军'' at line 1
 mysql> SELECT @w;
 +------+
 | @w   |
 +------+
 | NULL |
 +------+
 1 row in set (0.00 sec)
 
 mysql> SET @w := (SELECT COUNT(*) FROM course WHERE cteacher='程军');
 Query OK, 0 rows affected (0.00 sec)
 
 mysql> SELECT @w;
 +------+
 | @w   |
 +------+
 |    2 |
 +------+
 1 row in set (0.00 sec)
 
 mysql>

 SHOW TABLES;
 DROP TABLE IF EXISTS student;
 SHOW TABLES;
 CREATE TABLE IF NOT EXISTS student
 (
     sno int primary key,
     sname VARCHAR(20) CHARACTER SET gbk,
     sage TINYINT,
     shair ENUM('黑','白') CHARACTER SET gbk
 );
 INSERT INTO student VALUES('','李强',23,'黑');
 INSERT INTO student VALUES('','刘丽',22,'白');
 INSERT INTO student VALUES('','张友',22,'黑');
 SELECT * FROM student;
 
 SHOW TABLES;
 DROP TABLE IF EXISTS course;
 SHOW TABLES;
 CREATE TABLE IF NOT EXISTS course
 (
     cno VARCHAR(10) primary key,
     cname VARCHAR(20) CHARACTER SET gbk,
     cteacher  VARCHAR(20) CHARACTER SET gbk
 );
 INSERT INTO course VALUES('k1','c语言','王华');
 INSERT INTO course VALUES('k5','数据库原理','程军');
 INSERT INTO course VALUES('k8','编译原理','程军');
 SELECT * FROM course;
 
 SHOW TABLES;
 DROP TABLE IF EXISTS sc;
 SHOW TABLES;
 CREATE TABLE IF NOT EXISTS sc
 (
     fk_sno INT NOT NULL,
     fk_cno VARCHAR(10) NOT NULL,
     score INT NOT NULL,
     PRIMARY KEY(fk_sno,fk_cno),
     FOREIGN KEY(fk_sno) REFERENCES student(sno),
     FOREIGN KEY(fk_cno) REFERENCES course(cno)
 );
 INSERT INTO sc VALUES('','K1',83);
 INSERT INTO sc VALUES('','K1',85);
 INSERT INTO sc VALUES('','K1',92);
 INSERT INTO sc VALUES('','K5',90);
 INSERT INTO sc VALUES('','K5',84);
 INSERT INTO sc VALUES('','K8',80);
 SELECT * FROM sc;

1-检索至少选修＂程军＂老师所授全部课程的学生姓名（SNAME）
检索选修了“程军”老师所授全部课程的学生姓名

0-检索选修了“程军”老师所授课程的学生

 SELECT * FROM sc RIGHT JOIN course ON  sc.fk_cno=course.cno;
 SELECT * FROM (SELECT * FROM sc RIGHT JOIN course ON  sc.fk_cno=course.cno) w RIGHT JOIN student ON w.fk_sno=student.sno;
 SELECT * FROM (SELECT * FROM sc RIGHT JOIN course ON  sc.fk_cno=course.cno) w RIGHT JOIN student ON w.fk_sno=student.sno WHERE cteacher='程军';

集合子集，逐步精确，缩小范围 1-是学生-2-选程老师的课-3-选了程老师所有的课

/*

假定

“ 某学生选程老师的课程门数等于程老师所开门数”等价于“该学生至少选了程老师的所有的课程”。

*/

             SELECT sname FROM student
             WHERE
             (
                 SELECT COUNT(*) FROM
                 (
                     SELECT  * FROM (SELECT * FROM sc RIGHT JOIN course ON  sc.fk_cno=course.cno) w RIGHT JOIN student ON w.fk_sno=student.sno
                     WHERE cteacher = '程军'
                 ) wb
                 WHERE wb.sname = student.sname
             )
             =
             (SELECT COUNT(*) FROM course WHERE cteacher='程军');

/* 0-澄清问题：找出至少选了程老师全部课程的学生：学生-没有选程老师的课+学生选了程老师的部分的课+学生选了程老师全部的课

等价转化：找出选了程老师部分的课或者没有选程老师的课的学生

数据关系：学生和老师没有数据关系，是两个完全独立的数据对象，分数是桥梁

等价转化：

　　选了部分程老师的课 ，NOT EXISTS (SELECT * FROM sc WHERE sc.sno = student AND sc.cno=course.cno)->FALSE (内层SELECT不空，TRUE,退出)

　　没有选程老师的课 teacher='程军'->FALSE ,后边的不用看了(中层SELECT空，FALSE，退出)

　　外中内-三层SELECT，外层返回的是最终的结果

　　最外层选得的最终量result，学生姓名，它的措施是将各个学生名称带入，逐个检测

1-程序表达： */

 SELECT sname FROM student
 WHERE NOT EXISTS
 (
    SELECT * FROM course
    WHERE cteacher='程军' AND NOT EXISTS
    (
        SELECT * FROM sc
        WHERE sc.fk_sno = student.sno AND sc.fk_cno = course.cno
     )
 ) ;

2- 检索选修全部课程的学生姓名(SNAME)。

 SELECT sname FROM student
 WHERE
 (
     SELECT COUNT(*) FROM
     (
         SELECT  * FROM (SELECT * FROM sc RIGHT JOIN course ON  sc.fk_cno=course.cno) w RIGHT JOIN student ON w.fk_sno=student.sno
     ) wb
     WHERE wb.sname = student.sname
 )
 =
 (SELECT COUNT(*) FROM course );
 
 SELECT sname FROM student
 WHERE NOT EXISTS
 (
     SELECT * FROM course
     WHERE NOT EXISTS
     (
         SELECT * FROM sc
         WHERE sc.fk_sno = student.sno AND sc.fk_cno = course.cno
     )
 );
 
 SELECT * FROM course
 WHERE NOT EXISTS
 (
     SELECT 3>5
 );
 
 SELECT * FROM course
 WHERE  EXISTS
 (
     SELECT 3>5
 );

explain

 mysql> explain
     -> SELECT sname FROM student
     -> WHERE
     -> (
     ->  SELECT COUNT(*) FROM
     ->  (
     ->          SELECT  * FROM (SELECT * FROM sc RIGHT JOIN course ON  sc.fk_cno=course.cno) w RIGHT JOIN student ON w.fk_sno=student.sno
     ->  ) wb
     ->  WHERE wb.sname = student.sname
     -> )
     -> =
     -> (SELECT COUNT(*) FROM course );
 +----+--------------------+---------+------------+--------+----------------+---------+---------+-------------------+------+----------+------------------------------+
 | id | select_type        | table   | partitions | type   | possible_keys  | key     | key_len | ref               | rows | filtered | Extra                        |
 +----+--------------------+---------+------------+--------+----------------+---------+---------+-------------------+------+----------+------------------------------+
 |  1 | PRIMARY            | student | NULL       | ALL    | NULL           | NULL    | NULL    | NULL              |    3 |   100.00 | Using where                  |
 |  5 | SUBQUERY           | NULL    | NULL       | NULL   | NULL           | NULL    | NULL    | NULL              | NULL |     NULL | Select tables optimized away |
 |  2 | DEPENDENT SUBQUERY | student | NULL       | ALL    | NULL           | NULL    | NULL    | NULL              |    3 |    33.33 | Using where                  |
 |  2 | DEPENDENT SUBQUERY | sc      | NULL       | ref    | PRIMARY,fk_cno | PRIMARY | 4       | w0811.student.sno |    2 |   100.00 | Using index                  |
 |  2 | DEPENDENT SUBQUERY | course  | NULL       | eq_ref | PRIMARY        | PRIMARY | 12      | w0811.sc.fk_cno   |    1 |   100.00 | Using index                  |
 +----+--------------------+---------+------------+--------+----------------+---------+---------+-------------------+------+----------+------------------------------+
 5 rows in set, 2 warnings (0.00 sec)
 
 mysql>
 mysql> explain
     -> SELECT sname FROM student
     -> WHERE NOT EXISTS
     -> (
     ->  SELECT * FROM course
     ->  WHERE NOT EXISTS
     ->  (
     ->          SELECT * FROM sc
     ->          WHERE sc.fk_sno = student.sno AND sc.fk_cno = course.cno
     ->  )
     -> );
 +----+--------------------+---------+------------+--------+----------------+---------+---------+------------------------------------+------+----------+--------------------------+
 | id | select_type        | table   | partitions | type   | possible_keys  | key     | key_len | ref                                | rows | filtered | Extra                    |
 +----+--------------------+---------+------------+--------+----------------+---------+---------+------------------------------------+------+----------+--------------------------+
 |  1 | PRIMARY            | student | NULL       | ALL    | NULL           | NULL    | NULL    | NULL                               |    3 |   100.00 | Using where              |
 |  2 | DEPENDENT SUBQUERY | course  | NULL       | index  | NULL           | PRIMARY | 12      | NULL                               |    3 |   100.00 | Using where; Using index |
 |  3 | DEPENDENT SUBQUERY | sc      | NULL       | eq_ref | PRIMARY,fk_cno | PRIMARY | 16      | w0811.student.sno,w0811.course.cno |    1 |   100.00 | Using index              |
 +----+--------------------+---------+------------+--------+----------------+---------+---------+------------------------------------+------+----------+--------------------------+
 3 rows in set, 3 warnings (0.00 sec)
 
 mysql>

“带入遍历”，“重复循环”，这是上边代码原貌，而两种思路的区别仅仅是让计算机计算时怎样“重复次数”少一点。

http://blog.csdn.net/wolfofsiberian/article/details/39346781

发问:

0-语言只是工具而已，不是解决问题方法的源泉，怎么减少对接语法API的时间？