MySQL中计算不重复值的速度很慢。如何加快速度？

可以使用索引来加速此过程。让我们首先创建一个表 –

mysql> create table DemoTable1905
   (
   FirstName varchar(20),
   LastName varchar(20) ,
   INDEX F_L_Name(FirstName,LastName)
   );
Query OK, 0 rows affected (0.00 sec)

使用insert命令在表中插入一些记录 –

mysql> insert into DemoTable1905 values('John','Smith');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1905 values('John','Doe');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1905 values('Adam','Smith');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1905 values('John','Doe');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1905 values('Chris','Brown');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1905 values('Adam','Smith');
Query OK, 1 row affected (0.00 sec)

使用select语句显示表中的所有记录 –

mysql> select * from DemoTable1905;

这将产生下面的输出 –

+-----------+----------+
| FirstName | LastName |
+-----------+----------+
| Adam      |    Smith |
| Adam      |    Smith |
| Chris     |    Brown |
| John      |     Doe  |
| John      |     Doe  |
| John      |    Smith |
+-----------+----------+
6 rows in set (0.00 sec)

这是计算不同值的查询 –

mysql> select count(distinct FirstName) from DemoTable1905;

这将产生下面的输出 –

+---------------------------+
| count(distinct FirstName) |
+---------------------------+
|                         3 |
+---------------------------+
1 row in set (0.00 sec)

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