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MySQL Count Distinct值过程非常缓慢。如何扣紧？

2024-04-30 22:37:05 413

要加快此过程，可以使用INDEX。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)
   );

使用插入命令在表中插入一些记录-

mysql> insert into DemoTable1905 values('John','Smith');
mysql> insert into DemoTable1905 values('John','Doe');
mysql> insert into DemoTable1905 values('Adam','Smith');
mysql> insert into DemoTable1905 values('John','Doe');
mysql> insert into DemoTable1905 values('Chris','Brown');
mysql> insert into DemoTable1905 values('Adam','Smith');

使用select语句显示表中的所有记录-

mysql> select * from DemoTable1905;

这将产生以下输出-

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

这是查询不同值的查询-

mysql> select count(distinct FirstName) from DemoTable1905;

这将产生以下输出-

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

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MySQL Count Distinct值过程非常缓慢。如何扣紧？

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