mysql数据库条件语句怎么写

IF condition THEN
    -- 执行语句
ELSEIF another_condition THEN
    -- 执行另一组语句
ELSE
    -- 执行其他语句
END IF;

DELIMITER //

CREATE PROCEDURE GetUserType(IN userID INT)
BEGIN
    DECLARE userType VARCHAR(50);
    
    SELECT type INTO userType FROM users WHERE id = userID;
    
    IF userType = 'admin' THEN
        SELECT '管理员';
    ELSEIF userType = 'user' THEN
        SELECT '普通用户';
    ELSE
        SELECT '未知用户类型';
    END IF;
END //

DELIMITER ;

CASE 
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ...
    ELSE resultN
END;

SELECT id, name,
    CASE 
        WHEN age < 18 THEN '未成年'
        WHEN age BETWEEN 18 AND 60 THEN '成年'
        ELSE '老年'
    END AS age_group
FROM users;

SELECT column1, column2, ...
FROM table_name
WHERE condition;

-- 查询年龄大于30岁的用户
SELECT id, name, age FROM users WHERE age > 30;

-- 查询名字为'张三'的用户
SELECT * FROM users WHERE name = '张三';