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]';
