MySQL 保姆级教程（四）：过滤数据

第 6 章 过滤数据

6.1 使用 where 子句

输入: SELECT global_grants.USER,WITH_GRANT_OPTION FROM global_grants WHERE WITH_GRANT_OPTION = 'N';
输出: 
+------------------+-------------------+
| USER             | WITH_GRANT_OPTION |
+------------------+-------------------+
| mysql.infoschema | N                 |
| mysql.session    | N                 |
| mysql.session    | N                 |
| mysql.session    | N                 |
| mysql.session    | N                 |
| mysql.session    | N                 |
| mysql.session    | N                 |
| mysql.session    | N                 |
| mysql.sys        | N                 |
+------------------+-------------------+
分析: 这条语句从 global_grants 表中检索两个列，只返回  WITH_GRANT_OPTION 为 N 的行
注意: 同时使用 order by 和 where 语句时，应该让 order by 位于 where 之后，否则报错

6.2 WHERE 子句操作符

=               等于
<>              不等于
!=              不等于
<               小于
<=              小于等于
>               大于
>=              大于等于
BETWEEN AND      在指定的两个值之间
IS NULL          空值

6.2.1 检查单个值

输入: SELECT global_grants.USER,global_grants.PRIV FROM global_grants WHERE global_grants.USER = 'mysql.sys';
输出: 
+-----------+-------------+
| USER      | PRIV        |
+-----------+-------------+
| mysql.sys | SYSTEM_USER |
+-----------+-------------+
分析: 检查 where global_grants.USER = 'mysql.sys' 语句，它返回 global_grants.USER 值为 mysql.sys 的行

6.2.2 不匹配检查

输入: SELECT global_grants.USER,global_grants.PRIV FROM global_grants WHERE global_grants.USER != 'root';
输出: 
+------------------+----------------------------+
| USER             | PRIV                       |
+------------------+----------------------------+
| mysql.infoschema | SYSTEM_USER                |
| mysql.session    | BACKUP_ADMIN               |
| mysql.session    | CLONE_ADMIN                |
| mysql.session    | CONNECTION_ADMIN           |
| mysql.session    | PERSIST_RO_VARIABLES_ADMIN |
| mysql.session    | SESSION_VARIABLES_ADMIN    |
| mysql.session    | SYSTEM_USER                |
| mysql.session    | SYSTEM_VARIABLES_ADMIN     |
| mysql.sys        | SYSTEM_USER                |
+------------------+----------------------------+
分析: 检查 where global_grants.USER != 'root' 语句，它返回 global_grants.USER 值不为 root 的行

6.2.3 范围值检查

输入: SELECT server_cost.cost_name,server_cost.default_value FROM server_cost WHERE server_cost.default_value BETWEEN 0 AND 10;
输出: 
+------------------------------+---------------+
| cost_name                    | default_value |
+------------------------------+---------------+
| disk_temptable_row_cost      |           0.5 |
| key_compare_cost             |          0.05 |
| memory_temptable_create_cost |             1 |
| memory_temptable_row_cost    |           0.1 |
| row_evaluate_cost            |           0.1 |
+------------------------------+---------------+
分析: 在使用 BETWEEN 时，必须指定两个值------所需范围的最低值和最高值。这两个值必须用 AND 关键字分隔

6.2.4 空值检查

输入: SELECT server_cost.cost_name,server_cost.cost_value FROM server_cost WHERE server_cost.cost_value IS NULL;
输出: 
+------------------------------+------------+
| cost_name                    | cost_value |
+------------------------------+------------+
| disk_temptable_create_cost   |       NULL |
| disk_temptable_row_cost      |       NULL |
| key_compare_cost             |       NULL |
| memory_temptable_create_cost |       NULL |
| memory_temptable_row_cost    |       NULL |
| row_evaluate_cost            |       NULL |
+------------------------------+------------+
分析: IS NULL 语句可用来检查具有 NULL 值的列