MySQL——IF表达式把字段作为返回值讲解与实战

IF表达式 IF(expr1,expr2,expr3)

• 作为在SQL级别进行统一的if判断，处理特定场景逻辑

如果表达式expr1为true（且不等于0和Null），则if语句返回表达式expr2，否则返回表达式expr3，返回值是数字或字符串。强调的地方，表达式1、2、3：即可以是简单表达式，也可以是表字段。下面进行举例：

简单表达式返回

例如select *,if(gender=1,"男","女") as genderStr from student where gender != ''

把字段作为表达式返回值

例如select id,user_id,user_name,status,if(status = "Starting",min_price,max_price) as activityPrice from goods where 1=1

实战：实际业务SQL案例

SELECT
		td.id id,
		td.link_name linkName,
		td.link_type linkType,
		td.link_value linkValue,
		td.background_image backgroundImage,
		g.id goodsId,
		g.name as goodsName,
		g.preview_image previewImage,
		g.min_price minPrice,
		g.sale_begin_time saleBeginTime,
		g.sale_end_time saleEndTime,
		a.type activityType,
		// 把字段作为if表达式的返回值  
		if(a.status = 'STARTING',a.activity_min_price,g.min_price) as  activityMinPrice,
		a.status activityStatus
		FROM
		topic_detail td
		INNER JOIN goods g ON g.id = td.link_value
		INNER JOIN goods_scope gscp ON gscp.goods_id = g.id AND gscp.scope IN (1,441900,440000)
		LEFT JOIN activity a ON a.id = g.activity_id
		WHERE
		td.is_deleted = 0
		AND td.is_show = 1
		AND g.salable = 1
		AND g.deleted = 0
		AND g.status = 'SHELFING'
		AND CURRENT_TIMESTAMP between g.sale_begin_time and g.sale_end_time 
		AND td.topic_module_id = 99
	    ORDER BY  td.weight DESC