sql case when then

在SQL中，CASE语句用于根据条件返回不同的结果表达式。CASE语句的语法如下：

CASE
WHEN condition1 THEN result1
WHEN condition2 THEN result2
...
ELSE result
END

在这个语法中，condition1、condition2等是条件，result1、result2等是对应条件为真时返回的结果表达式。ELSE子句是可选的，用于指定当所有条件都不满足时的默认结果表达式。 [2 [3]

例如，在查询学生成绩时，可以使用CASE语句将分数转换为对应的等级：

SELECT student_name,
CASE
WHEN score >= 90 THEN '优秀'
WHEN score >= 80 THEN '良好'
WHEN score >= 70 THEN '中等'
WHEN score >= 60 THEN '及格'
ELSE '不及格'
END AS grade
FROM student_scores;

这个查询会根据学生的分数给出相应的等级，如果没有满足任何条件，就返回不及格。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
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