Beginning SQL

Chapter 5
NULLs and Strings
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NULL data causes unexpected results in strings just as it does in math calculations. This section takes you
through some of the effects of NULL data as it relates to string functions, and the following section looks
at how to deal with it. For example, the following SQL uses the CONCAT() function to detail which city a
person lives in:
SELECT FirstName, LastName, City, CONCAT( CONCAT(FirstName,’ ‘),
CONCAT(CONCAT(LastName, ‘ lives in ‘), City))
FROM MemberDetails;
If you’re using MS SQL Server or MS Access, the SQL is as follows:
SELECT FirstName, LastName, City, FirstName + ‘ ‘ + + LastName + ‘ lives in ‘ +
City
FROM MemberDetails;
Either statement provides the same results, shown here:
FirstName LastName City CONCAT( CONCAT(FirstName,’ ‘),
CONCAT(CONCAT(LastName, ‘ lives in ‘), City))
Katie Smith Townsville Katie Smith lives in Townsville
Steve Gee New Town Steve Gee lives in New Town
John Jones Orange Town John Jones lives in Orange Town
Jenny Jones Orange Town Jenny Jones lives in Orange Town
John Jackson Orange Town John Jackson lives in Orange Town
Jack Johnson Big City Jack Johnson lives in Big City
Seymour Botts Windy Village Seymour Botts lives in Windy Village
Susie Simons Townsville Susie Simons lives in Townsville
Jamie Hills Orange Town Jamie Hills lives in Orange Town
Stuart Dales Windy Village Stuart Dales lives in Windy Village
William Doors Big City William Doors lives in Big City
Doris Night Dover Doris Night lives in Dover
Catherine Hawthorn NULL NULL
Everything’s fine with the results until you hit the last one, which just says NULL, despite the other text
and the fact that FirstName and LastName contain values. The NULL value in City has propagated,
resulting in the whole result being NULL.
The next section looks at how to deal with this problem of NULL propagation to produce a more acceptable
result.