MySQL流程控制函数汇总分析讲解

# 如果expr1为true，则返回expr2，否则返回expr3
IF(expr1,expr2,expr3)

		// 比较最大数
        int a=10;
        int b=5;
        // 比较
        int bigNum=a>b?a:b;

SELECT IF(10>5,10,5) AS bigNum;

# 如果expr1不为NULL，则返回expr1，否则返回expr2
IFNULL(expr1,expr2)

SELECT IFNULL(null,'不空') AS notNull

SELECT IF(null IS NULL,null,'不空') AS notNull

		int week=3;
        switch (week){
            case 1:
                System.out.println("星期一");
                break;
            case 2:
                System.out.println("星期二");
                break;
            case 3:
                System.out.println("星期三");
                break;
            case 4:
                System.out.println("星期四");
                break;
            case 5:
                System.out.println("星期五");
                break;
            case 6:
                System.out.println("星期六");
                break;
            case 7:
                System.out.println("星期日");
                break;
            default:
                System.out.println("非法数据");
                break;
        }

# NOW()函数用于获取当前日期和时间，
# WEEKDAY(date)函数,表示返回date对应的工作日索引，
# 因为索引从0开始，所以加1；也可以不加1，将when的常量改为工作日索引也可
SELECT CASE WEEKDAY(NOW())+1
	WHEN 1 THEN '星期一'
	WHEN 2 THEN '星期二'
    WHEN 3 THEN '星期三'
    WHEN 4 THEN '星期四'
    WHEN 5 THEN '星期五'
    WHEN 6 THEN '星期六'
    WHEN 7 THEN '星期日'	
	ELSE '非法数据'
END AS `week`;

		int grade=87;
        if (grade>=90){
            System.out.println("优秀");
        }else if (grade>=80){
            System.out.println("良好");
        }else if (grade>=70){
            System.out.println("一般");
        }else if (grade>=60){
            System.out.println("及格");
        }else {
            System.out.println("不及格");
        }

SELECT id,`name`,chinese,
	CASE
		WHEN chinese>=90 THEN '优秀'
		WHEN chinese>=80 THEN '良好'
		WHEN chinese>=70 THEN '一般'
		WHEN chinese>=60 THEN '及格'
		ELSE '不及格'
	END AS `rank`
FROM student