Aug 02 2010

Some notes on SQL: 7 – Subqueries, combining selections and views

This is the seventh, and final, in a series of blog posts on SQL, the first covered creating a database, the second selecting information from a database, the third commands to modify the structure and contents of an existing database, the fourth, advanced selection. The fifth post covered database design. The sixth post covered multi-table database operations. This post covers subqueries and views. No claim of authority is made for these posts, they are mainly intended as my notes on the topic.
This is largely a wrapping up blog post to cover a couple of items.As an alternative to the joins described in a previous post, “subqueries” can often be used. A subquery is essentially an entire query embedded in another query. Subqueries can be used with UPDATE, INSERT and DELETE statements, whilst joins cannot. However, joins can be used to bring columns from multiple tables. There are no special keywords involved in creating a subquery. There is some discussion on the pros and cons of subqueries and joins here on Stackoverflow.

In an uncorrelated subquery the so-called inner query can be evaluated with no knowledge of the contents of the outer query. This expression contains an uncorrelated subquery:

UPDATE raw_candidate_data
SET    gender = ‘f’
WHERE  firstname IN (SELECT firstname
FROM   firstname_gender_lookup
WHERE  gender = ‘f’); 

The inner query is the clause in brackets, in this instance it is a shorthand way of building a list for an “IN” comparison. Often an inner query returns a single value, i.e. for an average or maximum in a list.

This expression contains a correlated subquery:

UPDATE raw_candidate_data
SET    party_id = (SELECT party_id
FROM   parties
WHERE  raw_candidate_data.party = parties.party_name); 

The inner query requires information from the outer query to work, this expression acts as a look up.

Complex queries can be given aliases using the VIEW keyword, for example:

CREATE VIEW web_designer AS SELECT mc.first_name, mc.last_name, mc.phone,
mc.email FROM my_contacts mc NATURAL JOIN job_desired jd WHERE jd.title=‘Web Designer’; 

Can subsequently be used by:

SELECT * FROM   web_designers; 

The view web_designers can be treated just as any other table.

The results of multiple select commands can be combined using the keywords: UNION, UNION ALL, INTERSECT and EXCEPT. UNION returns the distinct union of the results of all the selects, i.e. with no repeats, UNION ALL includes the repeats. INTERSECT returns items that are common to both SELECTs and EXCEPT returns those items that are returned by the first SELECT but not the second. The general form for these combination operations is as follows:

SELECT title FROM   job_current
SELECT title FROM   job_desired
SELECT title FROM   job_listings
ORDER  BY title; 

Each SELECT statement must return the same number of columns and each column must be coercible to the same datatype. Only a single ORDER BY statement for the set of SELECTs can be used.