MYSQL 的 SELECT 语句中如何做到判断字段为空

SELECT ifnull(字段A,字段B) FROM tbl ....

这里我想的是如果字段A为空就取字段B的值，可以这样写似乎不对，求教~

(SELECT (CASE 字段A WHEN NOT NULL THEN 字段A ELSE 字段B END) FROM tbl WHERE tb1.id=tb2.id) as name

这是完整语句，最终name无法实现我想要的效果

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用 COALESCE(A,B)
SELECT COALESCE(A,B) FROM tbl

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SELECT CASE
       WHEN BIRTHDAY IS NULL
         THEN PASSWORD
       ELSE BIRTHDAY
       END
FROM table_name;

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(SELECT (CASE 字段A WHEN IS NOT NULL THEN 字段A ELSE 字段B END) FROM tbl WHERE tb1.id=tb2.id) as name

IS NOT NULL

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select (case A when not null then A else B end) as result from tbl

去掉了is可以了