Question

i have the following mysql tables with some sample data.

1) table: ai_shipment

+----+---------------------+---------------------+------------------+
| id | booking_date        | loading_date        | container_number |
+----+---------------------+---------------------+------------------+
|  1 | 2012-08-03 00:00:00 | 2012-08-04 00:00:00 | ABB987987BBC6    |
|  2 | 2012-08-05 00:00:00 | 2012-08-07 00:00:00 | BHJKKU78786GH    |
+----+---------------------+---------------------+------------------+

2) table: ai_purchase_item

+----+---------+----------+-----------+-----------+
| id | item_id | quantity | cost      | rate      |
+----+---------+----------+-----------+-----------+
|  1 |       1 |       50 | 1200.0000 | 1355.0000 |
|  2 |       2 |       20 |  550.0000 |  675.0000 |
|  3 |       4 |       70 |   70.0000 |   70.0000 |
|  4 |       6 |       90 |   90.0000 |   90.0000 |
|  5 |       7 |       80 |   80.0000 |   80.0000 |
+----+---------+----------+-----------+-----------+

3) table: shipment_purchase_item

+----+-------------+------------------+
| id | shipment_id | purchase_item_id |
+----+-------------+------------------+
|  1 |           1 |                2 |
|  2 |           2 |                3 |
+----+-------------+------------------+

basically i am storing all the purchased items details in ai_purchase_item, ai_shipment stores the shipping details, and ai_shipment_purchase_item stores records of items that is shipped.

what i am trying to do is, i want to select all records from ai_purchase_item that is not shipped. which means the foreign key should not exist in ai_shipment_purchase_item.

with reference to above records the result i am expecting to be fetched is.

+----+---------+----------+-----------+-----------+
| id | item_id | quantity | cost      | rate      |
+----+---------+----------+-----------+-----------+
|  1 |       1 |       50 | 1200.0000 | 1355.0000 |
|  4 |       6 |       90 |   90.0000 |   90.0000 |
|  5 |       7 |       80 |   80.0000 |   80.0000 |
+----+---------+----------+-----------+-----------+

i tried something like this (i know the sql query is not proper)

 SELECT 
    pi.id, 
    pi.item_id, 
    pi.quantity, 
    pi.cost,
    pi.rate 
 FROM 
    ai_purchase_item pi 
 JOIN 
    ai_shipment_purchase_item spi ON spi.purchase_item_id = pi.id
 WHERE NOT EXISTS (SELECT spi.purchase_item_id WHERE spi.purchase_item_id = pi.id)

thank you.

Answer 1

There are three common approaches for finding values present in one table but missing in another in MySQL:

• LEFT JOIN
• NOT EXISTS
• NOT IN

Here’s the approach using NOT EXISTS:

SELECT 
    pi.id, 
    pi.item_id, 
    pi.quantity, 
    pi.cost,
    pi.rate 
FROM 
    ai_purchase_item AS pi 
WHERE NOT EXISTS
(
    SELECT *
    FROM ai_shipment_purchase_item AS spi
    WHERE spi.purchase_item_id = pi.id
)

And here’s a LEFT JOIN:

 SELECT 
    pi.id, 
    pi.item_id, 
    pi.quantity, 
    pi.cost,
    pi.rate 
 FROM 
    ai_purchase_item AS pi 
 LEFT JOIN 
    ai_shipment_purchase_item AS spi ON spi.purchase_item_id = pi.id
 WHERE spi.purchase_item_id IS NULL

And here’s NOT IN:

SELECT 
    pi.id, 
    pi.item_id, 
    pi.quantity, 
    pi.cost,
    pi.rate 
FROM 
    ai_purchase_item AS pi 
WHERE pi.id NOT IN
(
     SELECT purchase_item_id
     FROM ai_shipment_purchase_item
)

The following article by Quassnoi compares the performance of each of these approaches:

• NOT IN vs. NOT EXISTS vs. LEFT JOIN / IS NULL: MySQL

The conclusion is:

the best way to search for missing values in MySQL is using a LEFT JOIN / IS NULL or NOT IN rather than NOT EXISTS.