mysql 中怎么写判断

IF(condition, value_if_true, value_if_false)

SELECT IF(1 > 0, 'True', 'False'); -- 输出 'True'

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

SELECT 
    CASE 
        WHEN age < 18 THEN 'Minor'
        WHEN age >= 18 AND age < 65 THEN 'Adult'
        ELSE 'Senior'
    END AS age_group
FROM users;

IF condition THEN
    -- 执行语句
ELSE
    -- 执行其他语句
END IF;

DELIMITER //

CREATE PROCEDURE CheckAge(IN user_age INT)
BEGIN
    IF user_age < 18 THEN
        SELECT 'Minor';
    ELSE
        SELECT 'Adult or Senior';
    END IF;
END //

DELIMITER ;

CALL CheckAge(20); -- 输出 'Adult or Senior'