介绍六个有用的MySQL的SQL语句

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 表名 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 StopAND 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 tJOIN information_schema.key_column_usage kUSING (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ELECT 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.tablesGROUP BY table_schema ;