MySQL 常用的拼接语句汇总

前言：在MySQL中 CONCAT ()函数用于将多个字符串连接成一个字符串，利用此函数我们可以将原来一步无法得到的sql拼接出来，在工作中也许会方便很多，下面主要介绍下几个常用的场景。

注：适用于5.7版本 低版本可能稍许不同。

1.拼接查询所有用户

SELECT DISTINCT
  CONCAT(
    'User: \'',
    USER,
    '\'@\'',
    HOST,
    '\';'
  ) AS QUERY
FROM
  mysql.USER;
# 当拼接字符串中出现'时 需使用\转义符

2.拼接DROP table

SELECT
  CONCAT(
    'DROP table ',
    TABLE_NAME,
    ';'
  )
FROM
  information_schema. TABLES
WHERE
  TABLE_SCHEMA = 'test';

3.拼接kill连接

SELECT
  concat('KILL ', id, ';')
FROM
  information_schema. PROCESSLIST
WHERE
  STATE LIKE 'Creating sort index';

4.拼接创建数据库语句

SELECT
  CONCAT(
    'create database ',
    '`',
  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'
  );

5.拼接创建用户的语句

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'
  );
#有密码字符串哦 在其他实例执行 可直接创建出与本实例相同密码的用户

6.导出权限脚本 这个shell脚本也用到了拼接

#!/bin/bash 
#Function export user privileges 

pwd=yourpass 
expgrants() 
{ 
 mysql -B -u'root' -p${pwd} -N $@ -e "SELECT CONCAT( 'SHOW GRANTS FOR ''', user, '''@''', host, ''';' ) AS query FROM mysql.user" | \
 mysql -u'root' -p${pwd} $@ | \
 sed 's/\(GRANT .*\)/\1;/;s/^\(Grants for .*\)/-- \1 /;/--/{x;p;x;}' 
} 

expgrants > /tmp/grants.sql
echo "flush privileges;" >> /tmp/grants.sql

7.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 = 'test' order by DATA_LENGTH desc;

8.查找无主键表 这个没用到拼接 也分享出来吧

#查找某一个库无主键表
SELECT
table_schema,
table_name
FROM
  information_schema.TABLES
WHERE
  table_schema = 'test'
AND TABLE_NAME NOT IN (
  SELECT
    table_name
  FROM
    information_schema.table_constraints t
  JOIN information_schema.key_column_usage k USING (
    constraint_name,
    table_schema,
    table_name
  )
  WHERE
    t.constraint_type = 'PRIMARY KEY'
  AND t.table_schema = 'test'
);

#查找除系统库外 无主键表
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'
) ;

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