IF....ELSE in SQL

Every programming language has flow of control ability.  What I mean by this is the language allows you to check your data and make a decision based on what is returned.  One of the most popular ways to control the flow of your program is an IF/ELSE statement.  An IF/ELSE statement allows you to check a value and IF it matches a certain criteria, do something, ELSE do something else.

The basic look of an IF/ELSE statement looks like this:

IF expression
    do this
ELSE
    do this

Let's say that we are building a table of customer data.  Before we continue, we want to check and see if we have any customers in our table.  If we do, print one message or else we print another message:

IF (select count(*) from CustomerTable) > 0
    PRINT 'We have customers'
ELSE
    PRINT 'We dont have customers'

If your block statement after the IF or ELSE does more than one thing, you will need to insert a BEGIN and END at the beginning of each block.  Below is an example:

IF (select count(*) from CustomerTable) > 0
BEGIN
    PRINT 'We have customers'
    PRINT 'That is great'
END
ELSE
    PRINT 'We dont have customers'
    PRINT 'Isnt that sad?'
END

These are very simple examples.  You can do much more than print messages to the screen.  You can also update data, delete data, select data, and much more.  Below is one example:

IF (select count(*) from CustomerTable ) > 0
BEGIN
    delete from CustomerTable
    PRINT 'We just deleted our customers'
END
ELSE
BEGIN
    PRINT 'We dont have customers'
    PRINT 'Isnt that sad?'

END

To read more on IF/ELSE statements, go here: SQL IF/ELSE