mysql where[猿教程]

MYSQL WHERE|猿教程|

在本教程中，您将学习如何在 SELECT 语句中使用 MySQL WHERE 子句从结果集中过滤行。

MySQL WHERE 子句介绍

WHERE 子句允许您为查询返回的行指定搜索条件。下面显示了 WHERE 子句的语法：

SELECT select_list
FROMtable_name
WHEREsearch_condition;

search_condition 是使用逻辑运算符 AND、OR 和 NOT 的一个或多个表达式的组合。

在 MySQL 中，谓词是一个布尔表达式，其计算结果为 TRUE、FALSE 或 UNKNOWN。

SELECT 语句将包括结果集中满足 search_condition 的任何行。

除了 SELECT 语句之外，您还可以在 UPDATE 或 DELETE 语句中使用 WHERE 子句来指定要更新或删除哪些行。

当执行带有 WHERE 子句的 SELECT 语句时，MySQL 在 FROM 子句之后和 SELECT 和 ORDER BY 子句之前评估 WHERE 子句：

MySQL WHERE 子句示例

我们将使用示例数据库中的 employees 表进行演示。

使用带有相等运算符示例的 MySQL WHERE 子句

以下查询使用 WHERE 子句查找职位为'Sales Rep'的所有员工：

SELECT lastname, firstname, jobtitle
FROMemployees
WHEREjobtitle = 'Sales Rep';

+-----------+-----------+-----------+
| lastname  | firstname | jobtitle  |
+-----------+-----------+-----------+
| Jennings  | Leslie    | Sales Rep |
| Thompson  | Leslie    | Sales Rep |
| Firrelli  | Julie     | Sales Rep |
| Patterson | Steve     | Sales Rep |
| Tseng     | Foon Yue  | Sales Rep |
| Vanauf    | George    | Sales Rep |
| Bondur    | Loui      | Sales Rep |
| Hernandez | Gerard    | Sales Rep |
| Castillo  | Pamela    | Sales Rep |
| Bott      | Larry     | Sales Rep |
| Jones     | Barry     | Sales Rep |
| Fixter    | Andy      | Sales Rep |
| Marsh     | Peter     | Sales Rep |
| King      | Tom       | Sales Rep |
| Nishi     | Mami      | Sales Rep |
| Kato      | Yoshimi   | Sales Rep |
| Gerard    | Martin    | Sales Rep |
+-----------+-----------+-----------+
17 rows in set (0.00 sec)

在此示例中，SELECT 语句检查 employees 表的所有行并仅选择 jobTitle 列中值为 Sales Rep 的行。

使用带有 AND 运算符的 MySQL WHERE 子句

以下示例使用 WHERE 子句查找职务为Sales Rep且office codes为 1 的员工：


SELECT lastname, firstname, jobtitle,officeCode
FROMemployees
WHEREjobtitle = 'Sales Rep' AND officeCode = 1;

+----------+-----------+-----------+------------+
| lastname | firstname | jobtitle  | officeCode |
+----------+-----------+-----------+------------+
| Jennings | Leslie    | Sales Rep | 1          |
| Thompson | Leslie    | Sales Rep | 1          |
+----------+-----------+-----------+------------+
2 rows in set (0.00 sec)

在此示例中，WHERE 子句中的表达式使用 AND 运算符组合两个条件：

jobtitle = 'Sales Rep' AND officeCode = 1;

仅当两个表达式的计算结果都为 TRUE 时，AND 运算符才计算为 TRUE。因此，查询返回 jobTitle 列中值为 Sales Rep 且 officeCode 为 1 的行。

将 MySQL WHERE 子句与 OR 运算符一起使用

此查询查找职位为Sales Rep的员工或office code为 1 的员工：

SELECT lastName, firstName, jobTitle, officeCode
FROMemployees
WHEREjobtitle = 'Sales Rep' OR officeCode = 1
ORDER BY officeCode , jobTitle;

+-----------+-----------+--------------------+------------+
| lastName  | firstName | jobTitle           | officeCode |
+-----------+-----------+--------------------+------------+
| Murphy    | Diane     | President          | 1          |
| Bow       | Anthony   | Sales Manager (NA) | 1          |
| Jennings  | Leslie    | Sales Rep          | 1          |
| Thompson  | Leslie    | Sales Rep          | 1          |
| Firrelli  | Jeff      | VP Marketing       | 1          |
| Patterson | Mary      | VP Sales           | 1          |
| Firrelli  | Julie     | Sales Rep          | 2          |
| Patterson | Steve     | Sales Rep          | 2          |
| Tseng     | Foon Yue  | Sales Rep          | 3          |
| Vanauf    | George    | Sales Rep          | 3          |
| Bondur    | Loui      | Sales Rep          | 4          |
| Hernandez | Gerard    | Sales Rep          | 4          |
| Castillo  | Pamela    | Sales Rep          | 4          |
| Gerard    | Martin    | Sales Rep          | 4          |
| Nishi     | Mami      | Sales Rep          | 5          |
| Kato      | Yoshimi   | Sales Rep          | 5          |
| Fixter    | Andy      | Sales Rep          | 6          |
| Marsh     | Peter     | Sales Rep          | 6          |
| King      | Tom       | Sales Rep          | 6          |
| Bott      | Larry     | Sales Rep          | 7          |
| Jones     | Barry     | Sales Rep          | 7          |
+-----------+-----------+--------------------+------------+
21 rows in set (0.00 sec)

仅当表达式之一的计算结果为 TRUE 时，OR 运算符才计算结果为 TRUE：

jobtitle = 'Sales Rep' OR officeCode = 1

因此，查询返回任何职位为Sales Rep或officeCode 1 的员工。

将 MySQL WHERE 子句与 BETWEEN 运算符示例一起使用

如果值在值范围内，则 BETWEEN 运算符返回 TRUE：

expression BETWEEN low AND high

以下查询查找位于officeCode为 1 到 3 的办公室的员工：

SELECT firstName, lastName, officeCode
FROMemployees
WHEREofficeCode BETWEEN 1 AND 3
ORDER BY officeCode;

+-----------+-----------+------------+
| firstName | lastName  | officeCode |
+-----------+-----------+------------+
| Diane     | Murphy    | 1          |
| Mary      | Patterson | 1          |
| Jeff      | Firrelli  | 1          |
| Anthony   | Bow       | 1          |
| Leslie    | Jennings  | 1          |
| Leslie    | Thompson  | 1          |
| Julie     | Firrelli  | 2          |
| Steve     | Patterson | 2          |
| Foon Yue  | Tseng     | 3          |
| George    | Vanauf    | 3          |
+-----------+-----------+------------+
10 rows in set (0.00 sec)

将 MySQL WHERE 子句与 LIKE 运算符示例一起使用

如果值与指定的模式匹配，LIKE 运算符的计算结果为 TRUE。

要形成模式，请使用 % 和 _ 通配符。 % 通配符匹配任何零个或多个字符的字符串，而 _ 通配符匹配任何单个字符。

以下查询查找姓氏以字符串 'son' 结尾的员工：

SELECT firstName, lastName
FROMemployees
WHERElastName LIKE '%son'
ORDER BY firstName;

+-----------+-----------+
| firstName | lastName  |
+-----------+-----------+
| Leslie    | Thompson  |
| Mary      | Patterson |
| Steve     | Patterson |
| William   | Patterson |
+-----------+-----------+
4 rows in set (0.00 sec)

使用带有 IN 运算符示例的 MySQL WHERE 子句

如果值与列表中的任何值匹配，则 IN 运算符返回 TRUE。

value IN (value1, value2,...)

以下示例使用带有 IN 运算符的 WHERE 子句来查找位于office code 为 1 的办公室的员工。

SELECT firstName, lastName, officeCode
FROMemployees
WHEREofficeCode IN (1 , 2, 3)
ORDER BY officeCode;

+-----------+-----------+------------+
| firstName | lastName  | officeCode |
+-----------+-----------+------------+
| Diane     | Murphy    | 1          |
| Mary      | Patterson | 1          |
| Jeff      | Firrelli  | 1          |
| Anthony   | Bow       | 1          |
| Leslie    | Jennings  | 1          |
| Leslie    | Thompson  | 1          |
| Julie     | Firrelli  | 2          |
| Steve     | Patterson | 2          |
| Foon Yue  | Tseng     | 3          |
| George    | Vanauf    | 3          |
+-----------+-----------+------------+
10 rows in set (0.00 sec)

将 MySQL WHERE 子句与 IS NULL 运算符一起使用

要检查一个值是否为 NULL，请使用 IS NULL 运算符，而不是等于运算符 (=)。如果值为 NULL，则 IS NULL 运算符返回 TRUE。

value IS NULL

在数据库世界中，NULL 是一个标记，表示某个值丢失或未知。而且 NULL 不等于数字 0 或空字符串。

以下语句使用带有 IS NULL 运算符的 WHERE 子句来获取 reportsTo 列中的值为 NULL 的行：

SELECT lastName, firstName, reportsTo
FROMemployees
WHEREreportsTo IS NULL;

+----------+-----------+-----------+
| lastName | firstName | reportsTo |
+----------+-----------+-----------+
| Murphy   | Diane     |      NULL |
+----------+-----------+-----------+
1 row in set (0.01 sec)

将 MySQL WHERE 子句与比较运算符一起使用

下表显示了可用于形成 WHERE 子句中的表达式的比较运算符。

Operator	Description
=	等于。您可以将它与几乎任何数据类型一起使用。
<> 或 !=	不等于
<	小于。您通常将它与数字和日期/时间数据类型一起使用。
>	大于
<=	小于或等于
>=	大于或等于

以下查询使用不等于 (<>) 运算符来查找不是Sales Rep的所有员工：

SELECT lastname, firstname, jobtitle
FROMemployees
WHEREjobtitle <> 'Sales Rep';

+-----------+-----------+----------------------+
| lastname  | firstname | jobtitle             |
+-----------+-----------+----------------------+
| Murphy    | Diane     | President            |
| Patterson | Mary      | VP Sales             |
| Firrelli  | Jeff      | VP Marketing         |
| Patterson | William   | Sales Manager (APAC) |
| Bondur    | Gerard    | Sale Manager (EMEA)  |
| Bow       | Anthony   | Sales Manager (NA)   |
+-----------+-----------+----------------------+
6 rows in set (0.00 sec)

以下查询查找office code大于 5 的员工：

SELECT lastname, firstname, officeCode
FROMemployees
WHERE officecode > 5;

+-----------+-----------+------------+
| lastname  | firstname | officeCode |
+-----------+-----------+------------+
| Patterson | William   | 6          |
| Bott      | Larry     | 7          |
| Jones     | Barry     | 7          |
| Fixter    | Andy      | 6          |
| Marsh     | Peter     | 6          |
| King      | Tom       | 6          |
+-----------+-----------+------------+
6 rows in set (0.00 sec)

以下查询返回office code小于或等于 4 (<=4) 的员工：

SELECT lastname, firstname, officeCode
FROMemployees
WHERE officecode <= 4;

+-----------+-----------+------------+
| lastname  | firstname | officeCode |
+-----------+-----------+------------+
| Murphy    | Diane     | 1          |
| Patterson | Mary      | 1          |
| Firrelli  | Jeff      | 1          |
| Bondur    | Gerard    | 4          |
| Bow       | Anthony   | 1          |
| Jennings  | Leslie    | 1          |
| Thompson  | Leslie    | 1          |
| Firrelli  | Julie     | 2          |
| Patterson | Steve     | 2          |
| Tseng     | Foon Yue  | 3          |
| Vanauf    | George    | 3          |
| Bondur    | Loui      | 4          |
| Hernandez | Gerard    | 4          |
| Castillo  | Pamela    | 4          |
| Gerard    | Martin    | 4          |
+-----------+-----------+------------+
15 rows in set (0.00 sec)

总结

使用 WHERE 子句按条件过滤行
MySQL 在 FROM 子句之后和 SELECT 和 ORDER BY 子句之前评估 WHERE 子句。