使用WHERE子句

在SELECT语句中，数据根据WHERE子句中指定的搜索条件进行过滤。
WHERE子句在表名(FROM子句)之后给出

SELECT parent_category_id, name FROM help_category WHERE parent_category_id = 22;

在同时使用ORDERBY和WHERE子句时，应该让ORDER BY位于WHERE之后，否则将会产生错误

WHERE子句操作符

检查单个值

SELECT parent_category_id, name FROM help_category WHERE parent_category_id <= 22;

SELECT parent_category_id, name FROM help_category WHERE name= 'xml';
//MySQL在执行匹配时默认不区分大小写

不匹配检查

//列出不等于4 的name
SELECT parent_category_id, name FROM help_category WHERE parent_category_id <> 4;
SELECT parent_category_id, name FROM help_category WHERE parent_category_id != 4;

范围值检查

//BETWEEN 也会包括指定的那两个值
SELECT parent_category_id, name FROM help_category WHERE parent_category_id BETWEEN 1 AND 20;

空值检查

//可用来检查具有NULL值的列。这个WHERE子句就是IS NULL子句
SELECT user FROM user WHERE user_attributes IS NULL;