How can I use the result of an SQL select
statement as a column of another SQL select
statement?
For example:
select a, b,
(select t from other_table where other_table.id = my_table.a) as c
from my_table
If the result of other_table
is one record, there is no problem. But if it has more than one record, it is going to fail.
Use the results of a query as a field in another query. You can use a subquery as a field alias. Use a subquery as a field alias when you want to use the subquery results as a field in your main query. Note: A subquery that you use as a field alias cannot return more than one field.
I usually only use this kind of querying when counting. For example:
SELECT s.name,
(SELECT COUNT(*) FROM product as p WHERE p.store_id=s.id)
FROM store as s
In this case there is always only one record returned and the query will work. Please note that this is just an example and you should use a join instead of a sub-query here.
In your case you could use the TOP
keyword to limit the number of records in your sub-query to one:
select a, b,
(select TOP 1 t from other_table where other_table.id = my_table.a) as c
from my_table
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