mysql拼接两张表_巧用SQL拼接语句

前言：

在日常数据库运维过程中，可能经常会用到各种拼接语句，巧用拼接SQL可以让我们的工作方便很多，达到事半功倍的效果。本篇文章将会分享几个日常会用到的SQL拼接案例，类似的SQL还可以举一反三，探索出更多的可能性哦。

_注意_：适用于5.7版本，其他版本可能稍许不同。

1.CONCAT函数介绍

授人以鱼不如授人以渔，拼接SQL主要用到的是CONCAT函数，我们先来介绍下该函数的用法。

CONCAT(s1,s2...,sn) 是合并字符串函数，返回结果为连接参数产生的字符串，参数可以是一个或多个，若有任何一个参数为 NULL，则返回值为 NULL。当拼接字符串中有 ' 时，要用 \ 转义，貌似用两个单引号也行，不过还是推荐用 \ 转义，下面用几个示例来了解下CONCAT函数的用法。

mysql> SELECT CONCAT('MySQL','5.7'),CONCAT('MySQL',NULL),CONCAT('\'MySQL\'');
+-----------------------+----------------------+---------------------+
| CONCAT('MySQL','5.7') | CONCAT('MySQL',NULL) | CONCAT('\'MySQL\'') |
+-----------------------+----------------------+---------------------+
| MySQL5.7              | NULL                 | 'MySQL'             |
+-----------------------+----------------------+---------------------+

简单介绍完CONCAT函数的使用方法后，下面分享几个用到SQL拼接的场景，也许在你工作中会用到哦。

2.拼接查询所有用户

SELECT DISTINCT
  CONCAT(
    'User: \'',
    USER,
    '\'@\'',
    HOST,
    '\';'
  ) AS QUERY
FROM
  mysql.USER;

3.拼接创建用户的语句

# 有密码字符串 在其他实例执行 可直接创建出与本实例相同密码的用户
SELECT
  CONCAT(
    'create user \'',
    user,
    '\'@\'',
    Host,
    '\' IDENTIFIED BY PASSWORD \'',
    authentication_string,
    '\';'
  ) AS CreateUserQuery
FROM
  mysql.`user`
WHERE
  `User` NOT IN (
    'root',
    'mysql.session',
    'mysql.sys'
  );

# 这样拼接也可以 带有密码认证插件
SELECT
  CONCAT(
    'create user \'',
    user,
    '\'@\'',
    Host,
    '\' IDENTIFIED WITH \'',
    plugin,
    '\' AS \'',
    authentication_string,
    '\';'
  ) AS CreateUserQuery
FROM
  mysql.`user`
WHERE
  `User` NOT IN (
    'root',
    'mysql.session',
    'mysql.sys'
  );

4.拼接show grants语句查询用户权限

SELECT
  CONCAT(
    'show grants for \'',
    user,
    '\'@\'',
    Host,
    '\';'
  ) AS ShowGrants
FROM
  mysql.`user`
WHERE
  `User` NOT IN (
    'root',
    'mysql.session',
    'mysql.sys'
  );

5.拼接创建数据库语句

SELECT
  CONCAT(
    'create database if not exists ',
    '`',
    SCHEMA_NAME,
    '`',
    ' DEFAULT CHARACTER SET ',
    DEFAULT_CHARACTER_SET_NAME,
    ';'
  ) AS CreateDatabaseQuery
FROM
  information_schema.SCHEMATA
WHERE
  SCHEMA_NAME NOT IN (
    'information_schema',
    'performance_schema',
    'mysql',
    'sys'
  );

6.拼接DROP table

SELECT
  CONCAT(
    'DROP table ',
    TABLE_NAME,
    ';'
  )
FROM
  information_schema.TABLES
WHERE
  TABLE_SCHEMA = 'testdb' and TABLE_TYPE = 'BASE TABLE';

7.拼接kill连接

# 可以组合改变条件
SELECT
    concat( 'KILL ', id, ';' ) 
FROM
    information_schema.PROCESSLIST 
WHERE
    STATE LIKE 'Sending data';

SELECT
    concat( 'KILL ', id, ';' ) 
FROM
    information_schema.`PROCESSLIST` 
WHERE
    Command = 'Sleep' 
    AND TIME > 2000;

8.查看数据库大小

# 查看整个实例大小
SELECT
    concat( round( sum( data_length / 1024 / 1024 ), 2 ), 'MB' ) AS data_length_MB,
    concat( round( sum( index_length / 1024 / 1024 ), 2 ), 'MB' ) AS index_length_MB 
FROM
    information_schema.TABLES;

# 查看各个库大小
SELECT
    TABLE_SCHEMA,
    concat( TRUNCATE ( sum( data_length )/ 1024 / 1024, 2 ), ' MB' ) AS data_size,
    concat( TRUNCATE ( sum( index_length )/ 1024 / 1024, 2 ), 'MB' ) AS index_size 
FROM
    information_schema.TABLES 
GROUP BY
    TABLE_SCHEMA 
ORDER BY
    data_length DESC;

9.查找表碎片

SELECT t.TABLE_SCHEMA,
       t.TABLE_NAME,
       t.TABLE_ROWS,
     concat(round(t.DATA_LENGTH / 1024 / 1024, 2), 'M') AS size,
       t.INDEX_LENGTH,
       concat(round(t.DATA_FREE / 1024 / 1024, 2), 'M') AS datafree
FROM information_schema.tables t
WHERE t.TABLE_SCHEMA = 'testdb' order by DATA_LENGTH desc;

10.查找无主键表及增加自增ID作为主键

# 查找出无主键的表
SELECT
    t1.table_schema,
    t1.table_name
FROM
    information_schema.TABLES t1
LEFT OUTER JOIN information_schema.TABLE_CONSTRAINTS t2 ON t1.table_schema = t2.TABLE_SCHEMA
AND t1.table_name = t2.TABLE_NAME
AND t2.CONSTRAINT_NAME IN ('PRIMARY')
WHERE
    t2.table_name IS NULL
AND t1.TABLE_SCHEMA NOT IN (
    'information_schema',
    'performance_schema',
    'mysql',
    'sys'
) ;

# 拼接出增加自增ID作为主键的SQL
SELECT
CONCAT('ALTER TABLE ',t1.table_schema,'.',t1.table_name,' ADD COLUMN increment_id INT UNSIGNED NOT NULL auto_increment COMMENT \'自增主键\' PRIMARY KEY FIRST;')
FROM
    information_schema.TABLES t1
LEFT OUTER JOIN information_schema.TABLE_CONSTRAINTS t2 ON t1.table_schema = t2.TABLE_SCHEMA
AND t1.table_name = t2.TABLE_NAME
AND t2.CONSTRAINT_NAME IN ('PRIMARY')
WHERE
    t2.table_name IS NULL
AND t1.table_type = 'BASE TABLE'
AND t1.TABLE_SCHEMA NOT IN (
    'information_schema',
    'performance_schema',
    'mysql',
    'sys'
) ;

11.查找大写表及转为小写表

# 若lower_case_table_names=0可能导致表名既有大写又有小写，
# 想将lower_case_table_names设为1的话 需要先将大写的表和视图名称改为小写的。

# 查找出名称为大写的表和视图
SELECT
    TABLE_SCHEMA,
    TABLE_NAME,
    TABLE_TYPE 
FROM
    information_schema.`TABLES` 
WHERE
    TABLE_SCHEMA NOT IN ( 'information_schema', 'sys', 'mysql', 'performance_schema' ) 
    AND TABLE_NAME REGEXP BINARY '[A-Z]';

# 拼接出大写表名改为小写的SQL
SELECT
    CONCAT( 'rename table ', TABLE_SCHEMA, '.', TABLE_NAME, ' to ', TABLE_SCHEMA, '.', LOWER( TABLE_NAME ), ';' ) 
FROM
    information_schema.`TABLES` 
WHERE
    TABLE_SCHEMA NOT IN ( 'information_schema', 'sys', 'mysql', 'performance_schema' ) 
    AND TABLE_TYPE = 'BASE TABLE' 
    AND TABLE_NAME REGEXP BINARY '[A-Z]';