MySQL 查询多张表中类型相同字段的最大值

表结构

实现一：利用MySQL函数：GREATEST(value1,value2,...)

SELECT GREATEST(
	(SELECT max(price) from tb_goods),
	(SELECT max(price) from tb_goods2)
)

实现二：利用存储过程

-- 使用存储过程
delimiter @@
create PROCEDURE fun (OUT res VARCHAR(10))
BEGIN
	DECLARE m1 DECIMAL(6,2);
	DECLARE m2 DECIMAL(6,2);
	SELECT max(price) into m1 from tb_goods;
	SELECT max(price) into m2 from tb_goods2;
	if m1>m2 THEN
		SELECT m1 into res;
	ELSE
		SELECT m2 into res;
	END IF;
END @@

-- 调用存储过程
call fun(@ss);

-- 查看结果
select @ss