如何加快MySQL中的SELECT DISTINCT

要加快SELECT DISTINCT，可以在列或一组列上创建索引。让我们首先创建一个表 –

mysql> create table DemoTable
(
   Name varchar(40)
);
Query OK, 0 rows affected (1.13 sec)

以下是创建索引的查询 –

mysql> create index Name_Index on DemoTable(Name);
Query OK, 0 rows affected (1.56 sec)
Records: 0 Duplicates: 0 Warnings: 0

使用插入命令在表中插入一些记录 –

mysql> insert into DemoTable values('Chris');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable values('Mike');
Query OK, 1 row affected (0.15 sec)
mysql> insert into DemoTable values('Chris');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable values('Chris');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable values('Sam');
Query OK, 1 row affected (0.16 sec)

使用select语句显示表中的所有记录 –

mysql> select *from DemoTable;

这将产生以下输出 –

+-------+
| Name  |
+-------+
| Chris |
| Chris |
| Chris |
| Mike  |
| Sam   |
+-------+
5 rows in set (0.00 sec)

以下是仅选择不同记录的查询 –

mysql> select distinct Name from DemoTable;

这将产生以下输出 –

+-------+
| Name  |
+-------+
| Chris |
| Mike  |
| Sam   |
+-------+
3 rows in set (0.02 sec)

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