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WHERE 条件查询
WHERE用于指定查询的筛选条件，只返回满足条件的记录。它是SQL查询中最常用的子句之一，支持多种条件表达式。
sql
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-- 基本条件
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SELECT * FROM students WHERE age > 18;
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-- 精确匹配
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SELECT * FROM students WHERE name = '张三';
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-- 范围查询
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SELECT * FROM students WHERE score BETWEEN 60 AND 90;
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-- 集合查询
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SELECT * FROM students WHERE class_id IN (1, 3, 5);
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-- 模糊匹配
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SELECT * FROM students WHERE name LIKE '张%';
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SELECT * FROM students WHERE name LIKE '%明%';
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SELECT * FROM students WHERE name LIKE '_三';
📖WHERE后可跟多种条件表达式。常用运算符：=、!=、>、<、>=、<=、LIKE、IN、BETWEEN。LIKE中%匹配任意字符，_匹配单个字符。
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比较与逻辑运算
可用AND、OR、NOT组合多个条件，构建更精确的筛选逻辑。
sql
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-- AND：同时满足
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SELECT * FROM students
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WHERE age >= 18 AND gender = '男';
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-- OR：满足其一
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SELECT * FROM students
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WHERE class_id = 1 OR class_id = 2;
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-- NOT：取反
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SELECT * FROM students
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WHERE NOT gender = '女';
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-- 混合条件（括号优先）
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SELECT * FROM students
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WHERE (class_id = 1 OR class_id = 2)
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AND score > 80;
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-- NULL 判断
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SELECT * FROM students
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WHERE score IS NULL;
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SELECT * FROM students
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WHERE score IS NOT NULL;
✓AND为且，OR为或，NOT为非。混合使用多个条件时建议加括号明确优先级，避免歧义。IS NULL / IS NOT NULL 用于判断空值。