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分享6个有用的MySQL语句

2011-01-15 22:38:26| 分类：数据库 | 标签： |举报 |字号大中小订阅

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今天给大家介绍六条比较有用的MySQL的SQL语句，可能很多人都通过PHP来实现这些功能。

　　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. 查看你的数据库有多大

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 ;

　　希望对你有帮助。

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