当前位置：网站首页>MySQL tutorial 67- filter duplicate data using distinct

MySQL tutorial 67- filter duplicate data using distinct

2022-07-25 15:59:00 【51CTO】

stay MySQL Use in SELECT Statement when executing a simple data query , All matching records are returned . If some fields in the table have no uniqueness constraints , Then there may be duplicate values in these fields . In order to query non duplicate data ,MySQL Provides DISTINCT keyword .

DISTINCT The main function of keyword is to filter the duplicate data of one or more fields in the data table , Only one piece of data is returned to the user .

DISTINCT The syntax format of the keyword is ：

       
       SELECT DISTINCT < Field name > FROM < Table name >;
      
1.

among ,“ Field name ” Name the field to be de duplicated , Multiple fields are separated by commas .

Use DISTINCT You should pay attention to the following points when using keywords ：

DISTINCT Keywords can only be found in SELECT Use in statement .
When de duplicating one or more fields ,DISTINCT Keywords must be at the top of all fields .
If DISTINCT There are multiple fields after the keyword , Multiple fields will be combined to remove duplicate , in other words , Only when multiple fields are combined to be exactly the same will they be de duplicated .

example 1

The following is a specific example to illustrate how to query non duplicate data .

test In the database student The table structure and data of the table are as follows ：

       
       mysql> SELECT * FROM test.student;
       
+----+----------+------+-------+
       
| id | name     | age  | stuno |
       
+----+----------+------+-------+
       
|  1 | zhangsan |   18 |    23 |
       
|  2 | lisi     |   19 |    24 |
       
|  3 | wangwu   |   18 |    25 |
       
|  4 | zhaoliu  |   18 |    26 |
       
|  5 | zhangsan |   18 |    27 |
       
|  6 | wangwu   |   20 |    28 |
       
+----+----------+------+-------+
       
6 rows in set (0.00
      
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.

Results show ,student Presence in table 6 Bar record .

Following pair student Tabular age The fields are de duplicated ,SQL Statement and run results are as follows ：

       
       mysql> SELECT DISTINCT age FROM student;
       
+------+
       
| age  |
       
+------+
       
|   18 |
       
|   19 |
       
|   20 |
       
+------+
       
3 rows in set (0.00
      
1.
2.
3.
4.
5.
6.
7.
8.
9.

Yes student Tabular name and age The fields are de duplicated ,SQL Statement and run results are as follows ：

       
       mysql> SELECT DISTINCT name,age FROM student;
       
+----------+------+
       
| name     | age  |
       
+----------+------+
       
| zhangsan |   18 |
       
| lisi     |   19 |
       
| wangwu   |   18 |
       
| zhaoliu  |   18 |
       
| wangwu   |   20 |
       
+----------+------+
       
5 rows in set (0.00
      
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.

Yes student All fields in the table are de duplicated ,SQL Statement and run results are as follows ：

       
       mysql> SELECT DISTINCT * FROM student;
       
+----+----------+------+-------+
       
| id | name     | age  | stuno |
       
+----+----------+------+-------+
       
|  1 | zhangsan |   18 |    23 |
       
|  2 | lisi     |   19 |    24 |
       
|  3 | wangwu   |   18 |    25 |
       
|  4 | zhaoliu  |   18 |    26 |
       
|  5 | zhangsan |   18 |    27 |
       
|  6 | wangwu   |   20 |    28 |
       
+----+----------+------+-------+
       
6 rows in set (0.00
      
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.

because DISTINCT It can only return its target field , No other fields can be returned , So in reality , We use it a lot DISTINCT Keyword to return the number of non repeating fields .

Inquire about student The table is right name and age Number of records after field de duplication ,SQL Statement and run results are as follows ：

       
       mysql> SELECT COUNT(DISTINCT name,age) FROM student;
       
+--------------------------+
       
| COUNT(DISTINCT name,age) |
       
+--------------------------+
       
|                        5 |
       
+--------------------------+
       
1 row in set (0.01
      
1.
2.
3.
4.
5.
6.
7.

Results show ,student The table is right name and age After the field is de duplicated 5 Bar record .

原网站

版权声明
本文为[51CTO]所创，转载请带上原文链接，感谢
https://yzsam.com/2022/206/202207251532340412.html

当前位置：网站首页>MySQL tutorial 67- filter duplicate data using distinct

MySQL tutorial 67- filter duplicate data using distinct

example 1

边栏推荐

猜你喜欢

随机推荐