如何使用 COUNT(*) 返回单行而不是多行?
mysqlmysqli database
您需要使用 GROUP BY 和 COUNT(*) 来对值进行分组并显示计数以消除多个值。让我们首先创建一个表:
mysql> create table DemoTable (Value int); Query OK, 0 rows affected (0.55 sec)
下面是使用 insert 命令在表中插入一些记录的查询:
mysql> insert into DemoTable values(10); Query OK, 1 row affected (0.15 sec) mysql> insert into DemoTable values(20); Query OK, 1 row affected (0.13 sec) mysql> insert into DemoTable values(10); Query OK, 1 row affected (0.13 sec) mysql> insert into DemoTable values(30); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable values(10); Query OK, 1 row affected (0.11 sec) mysql> insert into DemoTable values(20); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable values(40); Query OK, 1 row affected (0.14 sec) mysql> insert into DemoTable values(10); Query OK, 1 row affected (0.18 sec) mysql> insert into DemoTable values(20); Query OK, 1 row affected (0.10 sec) mysql> insert into DemoTable values(20); Query OK, 1 row affected (0.07 sec) mysql> insert into DemoTable values(60); Query OK, 1 row affected (0.25 sec)
以下是使用 select 命令显示表中记录的查询:
mysql> select *from DemoTable;
这将产生以下输出:
+-------+ | Value | +-------+ | 10 | | 20 | | 10 | | 30 | | 10 | | 20 | | 40 | | 10 | | 20 | | 20 | | 60 | +-------+ 11 rows in set (0.00 sec)
以下查询返回类似记录的数量，在我们的例子中为 10、20、30、40、60:
mysql> SELECT COUNT(*) FROM (SELECT *FROM DemoTable GROUP BY Value) AS tbl;
这将产生以下输出:
+----------+ | COUNT(*) | +----------+ | 5 | +----------+ 1 row in set (0.00 sec)
