常用sql commands以及mysql问题解决日志

mysql workbench常用命令快捷键

ctrl+T ->创建新的sql query tab

ctrl+shift+enter->执行当前的sql命令

https://dev.mysql.com/doc/workbench/en/wb-keys.html

1. mysql -uroot -p

2. show databases;

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| ccpdev             |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
5 rows in set (0.00 sec)

3.exit;

4. create database intrepid_detectives; // 创建新的database

5. use intrepid_detectives; // 开始使用新创的那个数据库

database stores their data in tables.数据库将数据保存在table中，一个database可以有多张表,就像一个execel spreadsheet可以有多个sheet一样的概念。

6. show tables;

mysql> use intrepid_detectives;
Database changed
mysql> show tables;
Empty set (0.00 sec)

mysql> use mysql
Database changed
mysql> show tables;
+---------------------------+
| Tables_in_mysql           |
+---------------------------+
| columns_priv              |
| db                        |
| engine_cost               |
| event                     |
| func                      |
| general_log               |
| gtid_executed             |
| help_category             |
| help_keyword              |
| help_relation             |
| help_topic                |
| innodb_index_stats        |
| innodb_table_stats        |
| ndb_binlog_index          |
| plugin                    |
| proc                      |
| procs_priv                |
| proxies_priv              |
| server_cost               |
| servers                   |
| slave_master_info         |
| slave_relay_log_info      |
| slave_worker_info         |
| slow_log                  |
| tables_priv               |
| time_zone                 |
| time_zone_leap_second     |
| time_zone_name            |
| time_zone_transition      |
| time_zone_transition_type |
| user                      |
+---------------------------+
31 rows in set (0.00 sec)

从上面可以看出mysql默认builtin database内置有很多table

Every table has a set of columns, 每一张表都有一系列的columns(列).每一列都有一个name和一个datatype(at least).

7. create table <name> (<col-name> <col-type>,<col-name> <col-type>)

mysql> use intrepid_detectives;
Database changed
mysql> create table investigations (
    -> title varchar(100),
    -> detective varchar(30),
    -> daysToSolve integer);
Query OK, 0 rows affected (0.04 sec)

mysql> show tables;
+-------------------------------+
| Tables_in_intrepid_detectives |
+-------------------------------+
| investigations                |
+-------------------------------+
1 row in set (0.00 sec)

8. explain investigations; // 可以列出table的结构来：

mysql> explain investigations;
+-------------+--------------+------+-----+---------+-------+
| Field       | Type         | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+-------+
| title       | varchar(100) | YES  |     | NULL    |       |
| detective   | varchar(30)  | YES  |     | NULL    |       |
| daysToSolve | int(11)      | YES  |     | NULL    |       |
+-------------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

9. insert into investigations values("Finnigan's Falut","Carlotta McOwen", 4); //插入一行

table中的每一个entry被称为record. 一个table可以有多个records.  intersection of a column and a record is a field.

10. insert into investigations(datasToSolve, title,detective) values(3, "The Missing Tree", "Finch Hosky"); // 插入一行数据时，指定字段名称，不和数据库表中的字段顺序一致的办法

mysql> insert into investigations values("Finnigan's Falut","Carlotta McOwen", 4);
Query OK, 1 row affected (0.00 sec)
mysql> insert into investigations(daysToSolve, title,detective) values(3, "The Missing Tree", "Finch Hosky");
Query OK, 1 row affected (0.00 sec)

11. select * from investigations; // 从investigations表中获取所有数据

mysql> select * from investigations;
+------------------+-----------------+-------------+
| title            | detective       | daysToSolve |
+------------------+-----------------+-------------+
| Finnigan's Falut | Carlotta McOwen |           4 |
| The Missing Tree | Finch Hosky     |           3 |
| ohter ing Tree   | sssf            |           3 |
| ohter ing Tree   | sssf            |        NULL |
+------------------+-----------------+-------------+
4 rows in set (0.00 sec)

NULL is the SQL value for "no value";任何一个字段都可能是NULL，只要你不给他赋值！

12. select title, detective from investigations; // 只选择部分字段

13. data types:

• varchar(100) create table person(name varchar(100));
• text              create table email(body text);
• Numbers       int/integer: create table person(age integer unsigned);  // 正整数ff
• unsigned int: 0-4294967295
• BIGINT
• SMALLINT
• MEDIUMINT
• TINYINT
• decimal(precision, scope)比如decimal(10,0)=>0123546789; decimal(5,2)=>123.45;decimal(9,7)=>89.1234567
• auto_increment: 如果不给这个字段一个明确的值，则自动增1 create table student id integer auto_increment);
• date,time,datetime
• date: 'YYYY-MM-DD';
• time: '[H]HH:MM:SS'
• datetime: 'YYYY-MM-DD HH:MM:SS' create table order(order_date date);
• Booleans: bool/boolean: tinyint(1) create table order(fulfilled boolean); 0: false, no-0 is true
• default values:

create table order(coupon varchar(10) default "nodiscount", customer_id integer default null, datetime datetime default current_timestamp,fulfilled boolean NT NULL default 0);

每一个数据库table都必须有一个primary key, a column that quniquely identifies each row. it can ndeve be null  and must be set on record creation and never changed.

14. 创建constraint (主键)

mysql> create table detectives (
    -> id int not null auto_increment,
    -> name varchar(100),
    -> phone_number varchar(10),
    -> certificationDate date,
    -> constraint detectives_pk primary key (id));
Query OK, 0 rows affected (0.05 sec)

mysql> explain
    -> detectives;
+-------------------+--------------+------+-----+---------+----------------+
| Field             | Type         | Null | Key | Default | Extra          |
+-------------------+--------------+------+-----+---------+----------------+
| id                | int(11)      | NO   | PRI | NULL    | auto_increment |
| name              | varchar(100) | YES  |     | NULL    |                |
| phone_number      | varchar(10)  | YES  |     | NULL    |                |
| certificationDate | date         | YES  |     | NULL    |                |
+-------------------+--------------+------+-----+---------+----------------+

mysql> insert into detectives(name,phone_number,certificationDate)
    -> values("zhangsan",12333,"2002-01-03");
Query OK, 1 row affected (0.00 sec)

mysql> select * from detectives;
+----+----------+--------------+-------------------+
| id | name     | phone_number | certificationDate |
+----+----------+--------------+-------------------+
|  1 | zhangsan | 12333        | 2002-01-03        |
+----+----------+--------------+-------------------+
1 row in set (0.00 sec)

15. alter table investigations rename cases; // 修改表格名称

mysql> alter table investigations rename cases;
Query OK, 0 rows affected (0.01 sec)

mysql> show tables;
+-------------------------------+
| Tables_in_intrepid_detectives |
+-------------------------------+
| cases                         |
| detectives                    |
+-------------------------------+
2 rows in set (0.00 sec)

mysql>

16. alter table cases add criminal varchar(100) // 增加一列, drop criminal则删除一列

mysql> alter table cases add criminal varchar(100);
Query OK, 4 rows affected (0.04 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> select * from cases;
+------------------+-----------------+-------------+----------+
| title            | detective       | daysToSolve | criminal |
+------------------+-----------------+-------------+----------+
| Finnigan's Falut | Carlotta McOwen |           4 | NULL     |
| The Missing Tree | Finch Hosky     |           3 | NULL     |
| ohter ing Tree   | sssf            |           3 | NULL     |
| ohter ing Tree   | sssf            |        NULL | NULL     |
+------------------+-----------------+-------------+----------+
4 rows in set (0.00 sec)

17. alter table detectives change certificationDate certification_date date //更改 old certificationDate字段名称为new: certificate_date，type不变!!

mysql> alter table detectives change certificationDate certification_date date;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain detectives;
+--------------------+--------------+------+-----+---------+----------------+
| Field              | Type         | Null | Key | Default | Extra          |
+--------------------+--------------+------+-----+---------+----------------+
| id                 | int(11)      | NO   | PRI | NULL    | auto_increment |
| name               | varchar(100) | YES  |     | NULL    |                |
| phone_number       | varchar(10)  | YES  |     | NULL    |                |
| certification_date | date         | YES  |     | NULL    |                |
+--------------------+--------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

mysql> select * from cases;
+------------------+-----------------+-------------+----+
| title            | detective       | daysToSolve | id |
+------------------+-----------------+-------------+----+
| Finnigan's Falut | Carlotta McOwen |           4 |  1 |
| The Missing Tree | Finch Hosky     |           3 |  2 |
| ohter ing Tree   | sssf            |           3 |  3 |
| ohter ing Tree   | sssf            |        NULL |  4 |
| new case         | zhangsan        |           3 |  5 |
+------------------+-----------------+-------------+----+
5 rows in set (0.00 sec)

mysql> alter table cases change daysToSolve hours_to_solve int;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> select * from cases;
+------------------+-----------------+----------------+----+
| title            | detective       | hours_to_solve | id |
+------------------+-----------------+----------------+----+
| Finnigan's Falut | Carlotta McOwen |              4 |  1 |
| The Missing Tree | Finch Hosky     |              3 |  2 |
| ohter ing Tree   | sssf            |              3 |  3 |
| ohter ing Tree   | sssf            |           NULL |  4 |
| new case         | zhangsan        |              3 |  5 |
+------------------+-----------------+----------------+----+
5 rows in set (0.00 sec)

18. alter table cases add id int not null;

alter table cases add primary key (id); // 注意这时由于id默认为0，多个record都具有相同的id，因此这条命令会失败！！！

mysql> select * from cases;
+------------------+-----------------+-------------+----+
| title            | detective       | daysToSolve | id |
+------------------+-----------------+-------------+----+
| Finnigan's Falut | Carlotta McOwen |           4 |  0 |
| The Missing Tree | Finch Hosky     |           3 |  0 |
| ohter ing Tree   | sssf            |           3 |  0 |
| ohter ing Tree   | sssf            |        NULL |  0 |
+------------------+-----------------+-------------+----+
4 rows in set (0.00 sec)

mysql> alter table cases add primary key (id);
ERROR 1062 (23000): Duplicate entry '0' for key 'PRIMARY'

解决方案：需要手工或者程序将该列的值更改为不同的整数，随后才能够成功

下面是手工修正id为不同值后能够正确执行add prmiary key(id)和change id id int not null auto_increment的过程

mysql> select * from cases;
+------------------+-----------------+-------------+----+
| title            | detective       | daysToSolve | id |
+------------------+-----------------+-------------+----+
| Finnigan's Falut | Carlotta McOwen |           4 |  1 |
| The Missing Tree | Finch Hosky     |           3 |  2 |
| ohter ing Tree   | sssf            |           3 |  3 |
| ohter ing Tree   | sssf            |        NULL |  4 |
+------------------+-----------------+-------------+----+
4 rows in set (0.00 sec)

mysql> alter table cases add primary key(id);
Query OK, 4 rows affected (0.04 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> explain cases;
+-------------+--------------+------+-----+---------+-------+
| Field       | Type         | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+-------+
| title       | varchar(100) | YES  |     | NULL    |       |
| detective   | varchar(30)  | YES  |     | NULL    |       |
| daysToSolve | int(11)      | YES  |     | NULL    |       |
| id          | int(11)      | NO   | PRI | NULL    |       |
+-------------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table cases change id id INT NOT NULL AUTO_INCREMENT;
Query OK, 4 rows affected (0.04 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> explain cases;
+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| title       | varchar(100) | YES  |     | NULL    |                |
| detective   | varchar(30)  | YES  |     | NULL    |                |
| daysToSolve | int(11)      | YES  |     | NULL    |                |
| id          | int(11)      | NO   | PRI | NULL    | auto_increment |
+-------------+--------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

19. delete from criminals; // 删除整个criminals表格的数据

mysql> select * from criminals
    -> ;
+-----------+
| name      |
+-----------+
| crimianl1 |
| crimianl2 |
| crimianl3 |
+-----------+
3 rows in set (0.00 sec)

mysql> delete from criminals;
Query OK, 3 rows affected (0.00 sec)

mysql> select * from criminals;
Empty set (0.00 sec)

20. drop table criminals; // 删除criminals表

21. update <table> set <col>=<val> <conditions> ; //更新数据表记录

22. updat cases set hours_to_solve = hours_to_solve*24; // 我们将hours_to_solve这一列的值从天变换为小时,每一行都重新计算

注意的是有一行由于value为null,所以不会更改:

mysql> select * from cases;
+------------------+-----------------+----------------+----+
| title            | detective       | hours_to_solve | id |
+------------------+-----------------+----------------+----+
| Finnigan's Falut | Carlotta McOwen |              4 |  1 |
| The Missing Tree | Finch Hosky     |              3 |  2 |
| ohter ing Tree   | sssf            |              3 |  3 |
| ohter ing Tree   | sssf            |           NULL |  4 |
| new case         | zhangsan        |              3 |  5 |
+------------------+-----------------+----------------+----+
5 rows in set (0.00 sec)

mysql> update cases set hours_to_solve = hours_to_solve*24;
Query OK, 4 rows affected (0.00 sec)
Rows matched: 5  Changed: 4  Warnings: 0

mysql> select * from cases;
+------------------+-----------------+----------------+----+
| title            | detective       | hours_to_solve | id |
+------------------+-----------------+----------------+----+
| Finnigan's Falut | Carlotta McOwen |             96 |  1 |
| The Missing Tree | Finch Hosky     |             72 |  2 |
| ohter ing Tree   | sssf            |             72 |  3 |
| ohter ing Tree   | sssf            |           NULL |  4 |
| new case         | zhangsan        |             72 |  5 |
+------------------+-----------------+----------------+----+
5 rows in set (0.00 sec)

23. select * from cases where title = "xxx" // where clause限定选中的records:

select * from cases where title = "Finnigan's Falut";

select * from cases where detective_id !=2 ; //列出所有不是id为2的侦探处理的case

select * from cases where detective_id <>2 ; //列出所有不是id为2的侦探处理的case

select * from cases where start_date < '2015-02-02'; //列出早于2015-02-02的cases

select * from cases where detective_id =2 and hours_to_solve<90;

select * from cases where detective_id =1 or detective_id =2;

select * from cases where detective_id IN (1,2);

select * from cases where hours_to_solve between 24 and 90;

select * from detectives where first_name like "zhang%"

select * from detectives where phone_number NOT like "134"

24.  update cases set title="the man that wasnt" where title = "will be changed";

delete from cases where id = 4; 只删除4th

25.select * from cases limit 5;

26. select * from cases where start_date>'2010-01-01' limit 2;

27. select * from cases where start_date>'2010-01-01' limit 2 offset 4; 把前面的4个结果集descoped，只取2个

28. select distinct detective_id from cases; // 列出去重之后的detective_id的值

29. select distinct detective_id from cases where hours_to_solve > 50; // 列出去重之后的detective_id的值

30. select * from cases order by start_date desc;

31. select * from cases order by criminal_id DESC, start_date ASC; // 多个字段排序

32. select detective_id,AVG(hours_to_solve) from cases GROUP BY detective_id; //将结果集按照detective_id来group，并且对hours_to_solve字段来取平均数

select detective_id,avg(hours_to_solve) as avg_hours from cases group by detective_id order by avg_hours; 带orderby的groupby

33. select * from cases where detective_id = (select id from detectives where first_name="zhang");

select in select: 从detectives表中选择first_name为zhang的id作为另外一个select的条件

选择first_name为zhang的侦探他所有处理的case

34. select title from cases where detective_id IN (select id from detectives where phone_number like "8088%"); //选择所有phone_number以8088开头的侦探所处理的case的title

35. select first_name,last_name from detectives, cases where detectives.id = detective_id and criminial_id in (select id from criminals where first_name = "jim")

从detectives表中选中first/last_name，条件是detectives表的id字段等于 cases表的detective_id字段并且cases表的criminal_id字段在criminals表中的first_name为jim的结果集中

36. SELECT column_name(s) FROM table_name1 UNION SELECT column_name(s) FROM table_name2

union将两个select结果集合并(默认情况下去重),如果希望列出所有的则需要用UNION ALL关键字

37. select title, hours_to_solve as time from cases; // as关键字

38. select concat(first_name," ", last_name) as name from deteives // concat as

39. select d.first_name from detectives as d cross join criminals

foreign key is a column that links one table to another table by only accepting values that are already in a specified column of the second table: 外键

40. create table parent (id int, constraint parent_pk primary key (id));

create table child (parent int, constraint parent_fk foreign key (parent) references parent(id));

41. select count(*) from detectives where phone_number like "31%";

42. select 317 as area_code, count(*) as count from detectives where phone_number like "317%"

43: 从student表中找到每一个学生：名称，最小分数，最大分数（注意max,min实际上是通过test_score计算出来的，并不存在这一列）,并且groupby学生名

mysql> SELECT student_name, MIN(test_score), MAX(test_score)
    ->        FROM student
    ->        GROUP BY student_name;

44. count(*)

mysql> SELECT student.student_name,COUNT(*)
    ->        FROM student,course
    ->        WHERE student.student_id=course.student_id
    ->        GROUP BY student_name;

45. IF(exp1,exp2,exp3): 如果exp1为true返回exp2,否则返回exp3.

mysql> SELECT IF(1>2,2,3);
        -> 3
mysql> SELECT IF(1<2,'yes','no');
        -> 'yes'
mysql> SELECT IF(STRCMP('test','test1'),'no','yes');
        -> 'no'

46. RAND()函数

mysql> SELECT i, RAND() FROM t;
+------+------------------+
| i    | RAND()           |
+------+------------------+
|    1 | 0.61914388706828 |
|    2 | 0.93845168309142 |
|    3 | 0.83482678498591 |
+------+------------------+

47.mysql中的聚集类函数:

AVG() :返回平均值；

COUNT():返回行数

COUNT(DISTINCT):返回不同值的个数

GROUP_CONCAT():返回一个连接后的字符串

MAX()返回最大值；

MIN():返回最小值；

SUM():返回和值

注意上面的聚合类函数如果没有GROUP BY子句，将对所有的row执行聚合

mysql> SELECT student_name, AVG(test_score)
    ->        FROM student
    ->        GROUP BY student_name;

mysql> SELECT year, SUM(profit) FROM sales GROUP BY year WITH ROLLUP;
+------+-------------+
| year | SUM(profit) |
+------+-------------+
| 2000 |        4525 |
| 2001 |        3010 |
| NULL |        7535 |
+------+-------------+

mysql> SELECT year, country, product, SUM(profit)
    -> FROM sales
    -> GROUP BY year, country, product WITH ROLLUP;
+------+---------+------------+-------------+
| year | country | product    | SUM(profit) |
+------+---------+------------+-------------+
| 2000 | Finland | Computer   |        1500 |
| 2000 | Finland | Phone      |         100 |
| 2000 | Finland | NULL       |        1600 |
| 2000 | India   | Calculator |         150 |
| 2000 | India   | Computer   |        1200 |
| 2000 | India   | NULL       |        1350 |
| 2000 | USA     | Calculator |          75 |
| 2000 | USA     | Computer   |        1500 |
| 2000 | USA     | NULL       |        1575 |
| 2000 | NULL    | NULL       |        4525 |
| 2001 | Finland | Phone      |          10 |
| 2001 | Finland | NULL       |          10 |
| 2001 | USA     | Calculator |          50 |
| 2001 | USA     | Computer   |        2700 |
| 2001 | USA     | TV         |         250 |
| 2001 | USA     | NULL       |        3000 |
| 2001 | NULL    | NULL       |        3010 |
| NULL | NULL    | NULL       |        7535 |
+------+---------+------------+-------------+

48. User-defined variables:

mysql> SELECT @min_price:=MIN(price),@max_price:=MAX(price) FROM shop;
mysql> SELECT * FROM shop WHERE price=@min_price OR price=@max_price;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0003 | D | 1.25 |
| 0004 | D | 19.95 |
+---------+--------+-------+

我们可以在一个statement中保存一个user-defined variable,而在另外的statement中引用使用它。这允许你从一条查询传入value到另外一条查询。

User variable这样使用: @var_name, 不同使用中杠-，但是如果你希望使用-的话，可以用引号括起来@'my-var'

a)使用SET语句来创建user-defined variable(可以用=或者 := 作为赋值操作符)

SET @var_name = expr [, @var_name = expr] ...   // 或者使用:=赋值

b)也使用不用SET语句，而直接用 := 来赋值创建user-defined variable:

mysql> SET @t1=1, @t2=2, @t3:=4;
mysql> SELECT @t1, @t2, @t3, @t4 := @t1+@t2+@t3;
+------+------+------+--------------------+
| @t1 | @t2 | @t3 | @t4 := @t1+@t2+@t3 |
+------+------+------+--------------------+
| 1 | 2 | 4 | 7 |
+------+------+------+--------------------+

48. group by子句:

一个grouped table是在evaluate一个<group by clause>时产生的一系列groups。一个group G就是一系列row的集合，这些row具有以下特征： 每一个row的 grouping column GC,如果某行的GC字段的值为GV，那么这个group中的每一个GC字段的值都是GV;而且如果R1是grouped table GT的group G1里面的一个row的话， R2是GT里面的一个ROW并且：对每一个grouping column GC字段，R1的GC的值等于R2的GC的值的话，那么R2就必然在G1中出现。GT表中每一个row都只能在一个group中. 一个group就可以被认为是一个table. Set函数操作在groups之上。

49. on delete 如何实现父表记录一旦删除子表对应记录也能够删除

设想下面的场景： 一个buildings表，一个rooms表，一旦我们删除了building表一个记录，我们就应该删除对应rooms表中该building的rooms

CREATE TABLE buildings (
  building_no int(11) NOT NULL AUTO_INCREMENT,
  building_name varchar(255) NOT NULL,
  address varchar(355) NOT NULL,
  PRIMARY KEY (building_no)
) ENGINE=InnoDB;

CREATE TABLE rooms (
  room_no int(11) NOT NULL AUTO_INCREMENT,
  room_name varchar(255) NOT NULL,
  building_no int(11) NOT NULL,
  PRIMARY KEY (room_no),
  KEY building_no (building_no),
  CONSTRAINT rooms_ibfk_1
  FOREIGN KEY (building_no)
  REFERENCES buildings (building_no)
  ON DELETE CASCADE
) ENGINE=InnoDB;

INSERT INTO buildings(building_name,address)
VALUES('ACME Headquaters','3950 North 1st Street CA 95134'),
      ('ACME Sales','5000 North 1st Street CA 95134')
SELECT * FROM buildings;

INSERT INTO rooms(room_name,building_no)
VALUES('Amazon',1),
      ('War Room',1),
      ('Office of CEO',1),
      ('Marketing',2),
      ('Showroom',2)
SELECT * FROM rooms

DELETE FROM buildings
WHERE building_no = 2

SELECT * FROM rooms  //这时会将rooms表中对应building_no为2的记录全部删除

http://www.mysqltutorial.org/mysql-on-delete-cascade/

50. 如何找到on delete cascade对应会影响的表信息？

USE information_schema;

SELECT table_name
FROM referential_constraints
WHERE constraint_schema = 'classicmodels' AND
      referenced_table_name = 'buildings' AND
      delete_rule = 'CASCADE'

51. 如何查表中组合列出现的count次数？

SELECT id,difftag_id,difftaggable_id,difftaggable_type, count(*) as tags_count FROM difftaggables group by 3,4 order by tags_count

上面的sql代码中关键点为 group by 3,4 分别指difftaggable_id, difftaggable_type，这样就列出所有被某资源打过的difftag数量

select `id`, `title`,`root`,`level`, count(*) as dcount from cates group by 2,3 order by dcount desc

http://stackoverflow.com/questions/11072036/select-combination-of-two-columns-and-count-occurrences-of-this-combination

52. 如何计数得出表中组合列出现的unique次数

select count(*) from (
  select distinct folderid, userid from folder
)

http://stackoverflow.com/questions/8519747/sql-how-to-count-unique-combination-of-columns

53. 若干不能创建外键的错误解决办法：

http://stackoverflow.com/questions/11907147/struggling-adding-this-mysql-foreign-key

http://stackoverflow.com/questions/19137519/trouble-adding-foreign-key

很大的原因是表已经有了数据，而这些数据可能又不满足外键的约束，所以创建外键出错！注意这时你即使set @@global.foreign_key_check=0貌似不报错了，但是外键依然未创建成功！唯一的解决办法就是找出这些数据，手工删除或者解决外键不成功的问题。

SELECT * FROM `a` WHERE `a_id` not IN (SELECT id from b)

54. 查询某个字段为null

WHERE field IS NULL

55. Cannot drop index 'a_user_id_foreign': needed in a foreign key constraint解决办法

这时你需要先手工把foreign key 删除然后才能删除index!

alter table a drop foreign key key_name_here

注意foreign key 就是这样一堆东西: CONSTRAINT `ta_column_id_foreign` FOREIGN KEY (`column_id`) REFERENCES `b` (`id`) ON DELETE CASCADE ON UPDATE CASCADE, 而index就是KEY `ta_column_id_foreign` (`column_id`)

56. 如何查询到非unique的数据集

SELECT * from difftaggables WHERE difftaggable_id IN (SELECT difftaggable_id from (SELECT id,difftag_id,difftaggable_id,difftaggable_type, count(*) as tags_count FROM difftaggables group by 3,4 order by tags_count) AS hasduplicates WHERE tags_count > 1) ORDER by difftaggable_id

57. 关于key, primary key, foreign key，constrain等的概念

http://www.cnblogs.com/mydomain/archive/2011/11/10/2244233.html

http://www.cnblogs.com/Gile/p/MySQL.html

58. 复杂聚合函数以及子查询等的查询例子

select p1.* from homeworkuseranswers p1 inner join
(
  select max(created_at) as latest_date, count(user_id) as answer_count,
  user_id from homeworkuseranswers where homework_id=51 group by user_id

) p2
on p1.user_id = p2.user_id
and p1.created_at = p2.latest_date

59. Where in (id1,id2,id5,id3,id4)如何也orderby这个数组的value顺序?

在mysql查询中，我们可能有这样的要求：我们给定id数组列表，随后希望获取到按照在id数组列表中出现的顺序来获取到对应的model数据，但是由于mysql会自动按照id的从小到大的顺序来做排序，甚是烦人，总么办？

SELECT *
FROM target_table
WHERE id IN (4, 2, 6, 1)
ORDER BY field(id, 4, 2, 6, 1);

如果你用laravel,则可以翻译成:

$orderbyrawstring = 'field(id,' . implode(",", $ids).')';
YourModel::whereIn('id',$ids)->>orderByRaw($orderbyrawstring)->get();

60. SQLSTATE[HY000] [1862] Your password has expired. To log in you must change it using a client that supports expired passwords

mysql -uroot -p
mysql> set password=PASSWORD("xxxx");

http://www.omgdba.com/fixed-your-password-has-expired-to-log-in-you-must-change-it-using-a-client-that-supports-expired-passwords-on-mysql-5-7.html

MySQL5.7 中增加了密码过期机制，如果到期没有修改密码的话，会被数据库拒绝连接或进入沙箱模式（可以连进去，却无法操作）。但是总有些人会忘记定期修改密码这回事，等到过期了无法登陆却抓头搔耳无所适从，本文记录了一次“Your password has expired. To log in you must change it using a client that supports expired passwords”错误的修复过程，以供出现同样状况的道友参考。

1. 使用mysql 5.7 安装路径中的mysql程序登陆数据库，即~\mysql\bin\mysql.exe 文件。
2. 登陆后使用alter user ‘root’@’%’  identifity by ‘new-password’
3. 即可使用新密码登陆数据库
4. 如果不想让密码过期，可以使用以下方式：
   • 针对全局，可以修改配置文件，修改
      

     1 2	[mysqld] default_password_lifetime=0

   • 针对单个用户，可是在修改密码的时候加入PASSWORD EXPIRE NEVER;参数，形如：
      

     1	ALTER USER 'jeffrey'@'localhost' PASSWORD EXPIRE NEVER;

5. 以上。

更多详情，请参阅Password Expiration Policy。

61. count if怎么实现？

在sql查询中，有时我们可能要聚合列出一个字段，我们知道count函数是可以实现这个功能的，但是有的时候他不能完全满足要求。比如，我们要判断answer是正确的行数，这时简单使用count就无法满足了，这时我们可以使用SUM函数来曲线救国实现这个功能:

// 这条查询语句返回一个用户所有答对的习题聚合数目
select user_name,SUM(CASE isright WHEN 0 THEN 0 ELSE 1 END) AS right_count from answers 

62. 什么是corelated subquery?

a correlated subquery is a subquery that is executed once for each row

A correlated subquery returns results based on the column of the main query

select t1.*
from Table1 t1
where t1.id in
(
  select t2.id
  from Table2 t2
  where t2.value = t1.value
)

63. \ 等特殊符号在query中出现的处理办法

比如 where x_str = 'contains\\\\one back slash'

上面的字符串等价于contains\one back slash

64. 如何从值数组中临时形成一个表作为join的一方？

select * from (SELECT 1176 AS homeworkid, 117 AS uid
UNION ALL
SELECT 1175, 117
UNION ALL
SELECT 1174, 117) as temp1