前言：在MySQL中 CONCAT ()函式用於將多個字串連線成一個字串，利用此函式我們可以將原來一步無法得到的sql拼接出來，在工作中也許會方便很多，下面主要介紹下幾個常用的場景。

注：適用於5.7版本 低版本可能稍許不同。

1.拼接查詢所有使用者

SELECT DISTINCT
  CONCAT(
    'User: \'',USER,'\'@\'',HOST,'\';'
  ) AS QUERY
FROM
  mysql.USER;
# 當拼接字串中出現'時 需使用\轉義符

2.拼接DROP table

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

3.拼接kill連線

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

4.拼接建立資料庫語句

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.拼接建立使用者的語句

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'
  );
#有密碼字串哦 在其他例項執行 可直接創建出與本例項相同密碼的使用者

6.匯出許可權指令碼 這個shell指令碼也用到了拼接

#!/bin/bash 
#Function export user privileges 

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

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

7.查詢表碎片

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,'M') AS datafree
FROM information_schema.tables t
WHERE t.TABLE_SCHEMA = 'test' order by DATA_LENGTH desc;

8.查詢無主鍵表 這個沒用到拼接 也分享出來吧

#查詢某一個庫無主鍵表
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'
);

#查詢除系統庫外 無主鍵表
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','sys'
) ;

以上就是MySQL 常用的拼接語句彙總的詳細內容，更多關於MySQL 拼接語句的資料請關注我們其它相關文章！