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法律研究在 MySQL 中使用 GROUP BY 限制计数
我们首先创建一个表 -
mysql> create table DemoTable ( UserId int, UserMessage varchar(100) ); Query OK, 0 rows affected (0.51 sec)
使用 insert 命令在表中插入一些记录 -
mysql> insert into DemoTable values(1,'Hi'); Query OK, 1 row affected (0.09 sec) mysql> insert into DemoTable values(2,'Hello'); Query OK, 1 row affected (0.10 sec) mysql> insert into DemoTable values(2,'Good'); Query OK, 1 row affected (0.20 sec) mysql> insert into DemoTable values(1,'Nice'); Query OK, 1 row affected (0.18 sec) mysql> insert into DemoTable values(1,'Awesome'); Query OK, 1 row affected (0.16 sec) mysql> insert into DemoTable values(1,'Amazing'); Query OK, 1 row affected (0.11 sec) mysql> insert into DemoTable values(1,'Good Morning'); Query OK, 1 row affected (0.12 sec)
使用 select 语句从表中显示所有记录 -
mysql> select *from DemoTable;
这将产生以下输出 -
+--------+--------------+ | UserId | UserMessage | +--------+--------------+ | 1 | Hi | | 2 | Hello | | 2 | Good | | 1 | Nice | | 1 | Awesome | | 1 | Amazing | | 1 | Good Morning | +--------+--------------+ 7 rows in set (0.00 sec)
以下是使用 GROUP BY 限制计数的查询 -
mysql> select UserId, case when count(*) < 4 then count(*) else 'Greater Than 4' end as 'NumberOfMessage' from DemoTable group by UserId;
这将产生以下输出 -
+--------+-----------------+ | UserId | NumberOfMessage | +--------+-----------------+ | 1 | Greater Than 4 | | 2 | 2 | +--------+-----------------+ 2 rows in set (0.06 sec)
更新于: 26-Sep-2019
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