How to merge two tables before doing a inner join? - mysql

Question

TABLE user
- id
- username
- email
- passwd
TABLE user_external
- id
- username
- email
- passwd
TABLE orders
- id
- user_id
- product
I have two tables containg userinformation i want to merge in order to get usersname on a order.
my select only fetches the name from the user table (and leaves the ones from external blank), but not the user_external.
Is there a way to UNION users tables, and then do a left join?
select
orders.product,
merged_user_table.name as name
FROM orders o
LEFT JOIN merged_user_table user_assigned_to ON o.user_id = merged_user_table.id
WHERE
orders.id = 1

Answer 1

The following should work:
SELECT
orders.product
,merged_user_table.name as name
FROM orders o
LEFT JOIN (
SELECT id,username,email,passwd
FROM user
UNION ALL
SELECT id,username,email,passwd
FROM user_external
) merged_user_table user_assigned_to
ON o.user_id = merged_user_table.id
WHERE
orders.id = 1
I have assumed that the user and user_external tables' users are distinct. Using UNION without ALL will effectively perform a SELECT DISTINCT between the two selects in the subquery, which would be less efficient.

Answer 2

While you can use a sub query the get a unioned table, then join that to orders, this does mean the main join won't use an index.
As such I would probably use 2 separate queries, one joining orders with user and the other joining orders with user_external, and union the results together.
This should work (using an INNER JOIN rather than a LEFT OUTER JOIN - can't see why you are using a LEFT OUTER JOIN but there might be a reason). You have specified to bring back a column called name, but neither user table has such a column so not sure if you mean to bring back the user name or the table name - so the below example SQL brings back both.
SELECT orders.product,
user.username as name,
'user' as table_name
FROM orders o
INNER JOIN user
ON o.user_id = user.id
WHERE orders.id = 1
UNION ALL
SELECT orders.product,
user_external.username as name,
'user_external' as table_name
FROM orders o
INNER JOIN user_external
ON o.user_id = user_external.id
WHERE orders.id = 1