如何在sql查询中匹配两个不同的表？

Question

I have these two kinds of table below:

 Stocks Table          Order Table
-------------         -------------
| stockid   |         | orderid   |
| stockname |         | orderqty  |
| stockqty  |         | stockid   |
-------------         -------------

What I want to do is when orderqty is greater than stockqty I want to display a message saying that stocks are not enough, and when orderqty is less than stockqty then it is done.

The main concern here is to identify wether orderqty is less than or greater than stockqty .

Answer 1

Please see this DB Fiddle and let me know if it helps.

https://www.db-fiddle.com/f/hgEdnyeTTuxamXna9ELL3P/2

CREATE TABLE t_stock (
  stockid INT,
  stockname VARCHAR(50),
  stockqty BIGINT
);
INSERT INTO t_stock (stockid, stockname, stockqty) VALUES (1, 'salmon', 150);
INSERT INTO t_stock (stockid, stockname, stockqty) VALUES (2, 'asparagus', 275);
INSERT INTO t_stock (stockid, stockname, stockqty) VALUES (3, 'lemon', 300);


CREATE TABLE t_order (
  orderid INT,
  orderqty BIGINT,
  stockid INT
);
INSERT INTO t_order (orderid, orderqty, stockid) VALUES (13, 150, 3);
INSERT INTO t_order (orderid, orderqty, stockid) VALUES (14, 275, 2);
INSERT INTO t_order (orderid, orderqty, stockid) VALUES (15, 300, 1);


SELECT o.orderid
      ,o.stockid
      ,o.orderqty
      ,s.stockqty
      ,CONCAT(CASE WHEN o.orderqty > s.stockqty
                   THEN 'stocks are not enough for '
                   WHEN o.orderqty = s.stockqty
                   THEN 'stocks are just enough for '
                   ELSE 'we have plenty of stock for '
                   END, s.stockname) as stockqty_analysis
  FROM t_stock s INNER JOIN t_order o
    ON s.stockid = o.stockid;

Answer 2

SELECT o.orderid, 
       o.orderqty,
       s.stockqty,
       'stocks are not enough' AS message 
FROM orders o
JOIN stocks s ON o.stockid = s.stockid
WHERE o.orderqty < s.stockqty