What’s the difference between and Join and a Subquery (T-SQL)?

1 thought on “What’s the difference between and Join and a Subquery (T-SQL)?”

1. Its usually different ways of thinking or solving the same problem.

   A join will join two or more tables together by a field related to both tables (ie, relationship of primary and foreign keys). It is typically easy to understand.

   A subquery statement involves a SELECT statement that selects particular values from a table. The values that the select query selects is dependant upon the subquery. The subquery itself is another SELECT statement.

   IE)
   SELECT au_id, au_lname, au_fname
   FROM authors
   WHERE au_id NOT IN (
   SELECT authors.au_id
   FROM authors, titleauthor
   WHERE
   authors.au_id = titleauthor.au_id
   )

   The

   SELECT authors.au_id
   FROM authors, titleauthor
   WHERE
   authors.au_id = titleauthor.au_id

   here acts as the subquery.

   Notice that it is almost as if the query and its subquery are “joined” by au_id. The query will only return values for which au_id does NOT exist in the subquery.

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