Beginning SQL

Chapter 5
The syntax for the CAST() function consists of the expression to be cast and what data type you want to
cast it as:
CAST(expression AS data_type)
The code example at the beginning of this section doesn’t work with DB2 because ZipCode is a character
data type column, and DB2 doesn’t automatically try to convert the data type to a number, unlike
other systems such as SQL Server.
In order to rewrite the earlier example so that it works with IBM’s DB2, you need to cast the ZipCode
column as an int data type, as illustrated in the following SQL, which will work with all the database
systems including DB2:
SELECT ZipCode, MemberId, CAST(ZipCode AS int) / MemberId
FROM MemberDetails;
Note that CAST() works only with later versions of MySQL, from 4.02 onward, and that only some of the
data types can be cast. To make the preceding code work in MySQL, you need to change it to the following:
SELECT ZipCode, MemberId, CAST(ZipCode AS signed integer) / MemberId
FROM MemberDetails;
You can convert most data types to another data type, so long as the data can be converted. For example,
ZipCode contains digits, so conversion to an integer data type works fine. If you try converting ZipCode
to a date data type, as shown in the following SQL, you would receive errors because the values contained
in ZipCode simply can’t be converted to a date:
SELECT ZipCode, MemberId, CAST(ZipCode AS date)
FROM MemberDetails;
MS Access SQL doesn’t support CAST(), but its Visual Basic language does have a number of conversion
functions, which are beyond the scope of this book.
This section covered how to convert values of a specific data type to another data type. The next section
returns to the problematic question of the NULL data type.
Re-examining NULL
NULLs can be a bit problematic and can lead to unexpected results when used with functions, in comparisons,
or in math. Whereas Chapter 3 covered the concept of NULL being unknown data, this section covers
some of the issues that NULLs can throw up when used with math functions, string functions, and
concatenation. The section also introduces the NULLIF() and COALESCE() functions.
NULLs and Math
180
If NULL appears in any of your SQL math, then the result is always NULL. To illustrate this point, use the
following SQL to create a temporary table called MyTable and insert a few NULL values: