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
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
