Another way to improve the SQL Query to avoid union?

Question

User can search by Postcode (eg: L14, L15, L16) or Location from a textbox.

If user type in "Liverpool", it will find all the shops that are located in "Liverpool". If User type in the postcode (Eg: L15), it will search all the shops that do delivery in L15 postcode zone.

See the Tables below:

mysql> select * from shops;
+----+----------+-----------+----------+
| id | name     | location  | postcode |
+----+----------+-----------+----------+
|  1 | Shop One | Liverpool | L10      |
|  2 | Shop Two | Liverpool | L16      |
+----+----------+-----------+----------+

-

mysql> select * from shops_delivery_area;
+------------------+---------+----------+---------------+
| delivery_area_id | shop_id | postcode | delivery_cost |
+------------------+---------+----------+---------------+
|                1 |       1 | L10      |          1.50 |
|                2 |       1 | L11      |          0.00 |
|                3 |       1 | L12      |          1.00 |
|                4 |       1 | L13      |          1.00 |
|                5 |       2 | L10      |          2.50 |
|                6 |       2 | L16      |          0.00 |
|                7 |       2 | L28      |          0.00 |
+------------------+---------+----------+---------------+

SQL Query:

SELECT U.* FROM 
   ((SELECT DISTINCT shops.*, DA.delivery_cost, DA.postcode AS AreaPostcode FROM shops
             JOIN shops_delivery_area as DA on (DA.shop_id = shops.id)
   WHERE DA.postcode = "Liverpool")
  UNION
   (SELECT DISTINCT shops.*, DA.delivery_cost, DA.postcode AS AreaPostcode FROM shops
             JOIN shops_delivery_area as DA on  
                              (DA.shop_id = shops.id AND
                              DA.postcode = shops.postcode)
   WHERE shops.location = "Liverpool")) as U

-

Result - by Location (Liverpool):

+----+----------+-----------+----------+---------------+--------------+
| id | name     | location  | postcode | delivery_cost | AreaPostcode |
+----+----------+-----------+----------+---------------+--------------+
|  1 | Shop One | Liverpool | L10      |          1.50 | L10          |
|  2 | Shop Two | Liverpool | L16      |          0.00 | L16          |
+----+----------+-----------+----------+---------------+--------------+

Result - by Postcode (L12):

+----+----------+-----------+----------+---------------+--------------+
| id | name     | location  | postcode | delivery_cost | AreaPostcode |
+----+----------+-----------+----------+---------------+--------------+
|  1 | Shop One | Liverpool | L10      |          1.00 | L12          |
+----+----------+-----------+----------+---------------+--------------+

It appear to be working correctly... Is there other way to improve the SQL query shorter to avoid union or something?

Answer 1

Since all tables and selected columns are the same, you can simply do this:

  SELECT DISTINCT shops.*, DA.delivery_cost, DA.postcode AS AreaPostcode FROM shops
             JOIN shops_delivery_area as DA on DA.shop_id = shops.id
   WHERE (DA.postcode = "Liverpool")
      OR (DA.postcode = shops.postcode AND shops.location = "Liverpool")

Like you said in Diego's answer, the conditions are a litle different! So, you compensate that difference in the WHERE clause .

Answer 2

Whatever you choose, be aware that short code is not always optimal code. In many cases, where you have sufficiently divergent logic, unioning the results really is the most optimal (and sometimes most clean, programatically) option.

That said, the following OR in the WHERE clause seems to cover both your cases...

SELECT DISTINCT
  shops.*,
  DA.delivery_cost,
  DA.postcode AS AreaPostcode
FROM
  shops
INNER JOIN
  shops_delivery_area as DA
    ON (DA.shop_id = shops.id)
WHERE
  (DA.postcode = "Liverpool")
OR
  (DA.postcode = shops.postcode AND shops.location = "Liverpool")

Answer 3

What am I missing? Why cant you do

 WHERE DA.postcode = "Liverpool" or shops.location = "Liverpool"

Answer 4

please try this:

SELECT DISTINCT shops.*, 
       DA.delivery_cost, 
       DA.postcode 
FROM shops 
       JOIN shops_delivery_area as DA on DA.shop_id = shops.id
WHERE DA.postcode = "Liverpool" 
      OR (location = "Liverpool" and DA.postcode = shops.postcode)