有用的MySQL语句

摘自onefish资料库

1. 计算年数
你想通过生日来计算这个人有几岁了。

SELECT DATE_FORMAT(FROM_DAYS(TO_DAYS(now()) - TO_DAYS(@dateofbirth)), '%Y') + 0; 

2. 两个时间的差
取得两个 datetime 值的差。假设 dt1 和 dt2 是 datetime 类型，其格式为 ‘yyyy-mm-dd hh:mm:ss’，那么它们之间所差的秒数为：

 UNIX_TIMESTAMP( dt2 ) - UNIX_TIMESTAMP( dt1 )

除以60就是所差的分钟数，除以3600就是所差的小时数，再除以24就是所差的天数。

3. 显示某一列出现过N次的值

SELECT id
FROM tbl
GROUP BY id
HAVING COUNT(*) = N; 

4. 计算两个日子间的工作日
所谓工作日就是除出周六周日和节假日。

SELECT COUNT(*)
FROM calendar
WHERE d BETWEEN Start AND Stop
  AND DAYOFWEEK(d) NOT IN(1,7)
  AND holiday=0;

5. 查找表中的主键

 SELECT k.column_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='db'
  AND t.table_name=tbl'
 

6. 查看你的数库有多大

 SELECT
  table_schema AS 'Db Name',
  Round( Sum( data_length + index_length ) / 1024 / 1024, 3 ) AS 'Db Size (MB)',
  Round( Sum( data_free ) / 1024 / 1024, 3 ) AS 'Free Space (MB)'
FROM information_schema.tables
GROUP BY table_schema ;

7.当你想要用字符串当成数字类型作为order by的字段时

　　

SELECT * FROM `uchome_plug_collect_in` WHERE `collectid` = 4744346 ORDER BY CAST(unote AS SIGNED) DESC
SELECT * FROM `uchome_plug_collect_in` WHERE `collectid` = 4744346 ORDER BY `unote`+0 DESC

8.查询数据大小

SELECT CONCAT( ROUND( ( INDEX_LENGTH + DATA_LENGTH ) /1024 /1024, 2 ) , 'MB' ) AS '总大小', CONCAT( ROUND( DATA_LENGTH /1024 /1024, 2 ) , 'MB' ) AS '数据大小', CONCAT( ROUND( INDEX_LENGTH /1024 /1024, 2 ) , 'MB' ) AS '索引大小' FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = '数据库名' AND TABLE_NAME = '表名'