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在MariaDB中，WHERE子句与SELECT，INSERT，UPDATE和DELETE语句一起使用来选择或更改想要更改的满足指定特定行记录。

它是一个在表名后面出现的语句。

语法

[COMMAND] field,field2,... FROM table_name,table_name2,... WHERE [CONDITION]

注意：WHERE子句是一个可选的子句。它可以和AND，OR，AND & OR，LIKE运算符一起使用。

1. WHERE子句与单一条件

示例：

我们有一个表students，里边有一些数据。假设要查询students表中的student_id小于6的所有记录。

SELECT *   
FROM Students  
WHERE student_id < 6;

执行上面查询语句，得到以下结果 -

MariaDB [testdb]> SELECT *
    -> FROM Students
    -> WHERE student_id < 6;
+------------+--------------+-----------------+----------------+
| student_id | student_name | student_address | admission_date |
+------------+--------------+-----------------+----------------+
|          1 | Maxsu        | Haikou          | 2017-01-07     |
|          3 | JMaster      | Beijing         | 2016-05-07     |
|          4 | Mahesh       | Guangzhou       | 2016-06-07     |
|          5 | Kobe         | Shanghai        | 2016-02-07     |
+------------+--------------+-----------------+----------------+
4 rows in set (0.00 sec)

2. WHERE子句与AND条件

为了方便演示，这里再插入一条数据 -

INSERT INTO students  
(student_name, student_address, admission_date)  
VALUES('Maxsu','Sanya','2017-08-08 00:00:00');

当前students表中的全部数据是 -

MariaDB [testdb]> select * from students;
+------------+--------------+-----------------+----------------+
| student_id | student_name | student_address | admission_date |
+------------+--------------+-----------------+----------------+
|          1 | Maxsu        | Haikou          | 2017-01-07     |
|          3 | JMaster      | Beijing         | 2016-05-07     |
|          4 | Mahesh       | Guangzhou       | 2016-06-07     |
|          5 | Kobe         | Shanghai        | 2016-02-07     |
|          6 | Blaba        | Shengzhen       | 2016-08-07     |
|          7 | Maxsu        | Sanya           | 2017-08-08     |
+------------+--------------+-----------------+----------------+
6 rows in set (0.00 sec)

现在要查询students表中的所有列，其中student_name是Maxsu，并且student_id大于1的行记录。这两个条件都必须满足。参考以下查询语句 -

SELECT *  
FROM Students  
WHERE student_name = 'Maxsu'  
AND student_id >1 ;

执行上面查询语句，得到以下结果 -

MariaDB [testdb]> SELECT *
    -> FROM Students
    -> WHERE student_name = 'Maxsu'
    -> AND student_id >1 ;
+------------+--------------+-----------------+----------------+
| student_id | student_name | student_address | admission_date |
+------------+--------------+-----------------+----------------+
|          7 | Maxsu        | Sanya           | 2017-08-08     |
+------------+--------------+-----------------+----------------+
1 row in set (0.03 sec)

3. WHERE子句与OR条件

现在要查询students表中的所有列，其中student_name是Maxsu，或者student_id大于5的行记录。这两个条件满足其中一个即可。参考以下查询语句 -

SELECT *  
FROM Students  
WHERE student_name = 'Maxsu'
OR student_id >5 ;

执行上面查询语句，得到以下结果 -

MariaDB [testdb]> SELECT *
    -> FROM Students
    -> WHERE student_name = 'Maxsu'
    -> OR student_id >5 ;
+------------+--------------+-----------------+----------------+
| student_id | student_name | student_address | admission_date |
+------------+--------------+-----------------+----------------+
|          1 | Maxsu        | Haikou          | 2017-01-07     |
|          6 | Blaba        | Shengzhen       | 2016-08-07     |
|          7 | Maxsu        | Sanya           | 2017-08-08     |
+------------+--------------+-----------------+----------------+
3 rows in set (0.00 sec)

4. WHERE子句与OR/AND条件

假设要查询students表中的所有列，那些符合student_name是Ajeet，student_id大于或等于1，或者student_address是Shanghai的行记录。

SELECT *  
FROM Students  
WHERE student_name = 'Maxsu'   
AND student_id >=1
OR (student_address = 'Shanghai');

执行上面查询语句，得到以下结果 -

MariaDB [testdb]> SELECT *
    -> FROM Students
    -> WHERE student_name = 'Maxsu'
    -> AND student_id >=1
    -> OR (student_address = 'Shanghai');
+------------+--------------+-----------------+----------------+
| student_id | student_name | student_address | admission_date |
+------------+--------------+-----------------+----------------+
|          1 | Maxsu        | Haikou          | 2017-01-07     |
|          5 | Kobe         | Shanghai        | 2016-02-07     |
|          7 | Maxsu        | Sanya           | 2017-08-08     |
+------------+--------------+-----------------+----------------+
3 rows in set (0.00 sec)

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