如何选择具有空值的列并使用列的id来选择另一个表的另一列

Question

consider table req and orders

CREATE TABLE `req` (
  `req_id` bigint(20) NOT NULL,
  `order_date` timestamp NULL DEFAULT NULL,
  `status` varchar(20) COLLATE utf8_bin DEFAULT NULL,
  `order_id` bigint(20) NOT NULL,`
  PRIMARY KEY (`req_id`),
  KEY `order_id` (`order_id`),
  CONSTRAINT `req_ibfk_16` FOREIGN KEY (`order_id`) REFERENCES `order` (`order_id`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
/*!40101 SET character_set_client = @saved_cs_client */;


CREATE TABLE `orders` (
  `order_id` bigint(20) NOT NULL,
  `acc_id` bigint(20) DEFAULT NULL,
  `acc_name` varchar(256) COLLATE utf8_bin DEFAULT NULL,
  PRIMARY KEY (`order_id`),
  KEY `index_name` (`order_id`,`account_id`),
  CONSTRAINT `order_ibfk_1`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
/*!40101 SET character_set_client = @saved_cs_client */;

"To select column oder_id, oder_date and status where status is null value of table req and use the oder id to select columns acc_id and acc_name of orders table" ?

and diplay them jointly as

status , oder_id , acc_id, acc_name

SELECT status,order_date,order_id FROM req WHERE status is null;

but i have problem on how to select another table column using the id and show them jointly

i have used this for select column from one table

SELECT status,order_date,order_id FROM req WHERE status is null;

i expect a output of select columns mentioned below

status , oder_id , acc_id, acc_name

Answer 1

You need to join the two table by using the relationship that they have Based on your table, order_id is primary key of 'orders' and is the FK of 'req'

Use this code:

SELECT 
    req.status , req.oder_id , orders.acc_id, orders.acc_name
FROM 
    req, orders
WHERE 
    req.order_id = orders.order_id
and
    req.status is null;

This basically joins the two tables and you need to write what table the fields where come from. This is the long version of the code but its the easiest way to understand.

let me know if you have any questions.

Edit: Adding another approach by not Comma separated join. Use full join for instead.

SELECT 
    req.status , req.oder_id , orders.acc_id, orders.acc_name
FROM 
    req 
Full Join
    orders
on 
    req.order_id = orders.order_id
WHERE 
    req.status is null;