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1. 查询时间

1) timestamp 日期类型
     $sql = "select date_format(create_time, '%Y-%m-%d') as day from table_name";
2) int 时间戳类型
     $sql = "select from_unixtime(create_time, '%Y-%m-%d') as day from table_name";

2. 一个sql 返回多个总数

$sql = "select count(*) all, " ;
$sql .= " count(case when status = 1 then status end) status_1_num, " ; 
$sql .= " count(case when status = 2 then status end) status_2_num ";
$sql .= " from table_name";

3. Update Join / Delete Join

$sql = "update table_name_1 ";
$sql .= " inner join table_name_2 on table_name_1.id = table_name_2.uid ";
$sql .= " inner join table_name_3 on table_name_3.id = table_name_1.tid ";
$sql .= " set *** = *** ";
$sql .= " where *** ";
//delete join 同上。

4. 替换某字段的内容的语句

$sql = "update table_name set content = REPLACE(content, 'aaa', 'bbb') ";
$sql .= " where (content like '%aaa%')";

5. 获取表中某字段包含某字符串的数据

$sql = "SELECT * FROM `表名` WHERE LOCATE('关键字', 字段名) ";

6. 查找表中多余的重复记录

1) 单个字段
     $sql = "select * from 表名 where 字段名 in ";
     $sql .= "(select 字段名 from 表名 group by 字段名 having count(字段名) > 1 )";
2) 多个字段
     $sql = "select * from 表名 别名 where (别名.字段1,别名.字段2) in ";
     $sql .= "(select 字段1,字段2 from 表名 group by 字段1,字段2 having count(*) > 1 )";