Treat empty variable - Excel VBA

[Alvim]

Dear,

I would like to know how to treat an empty variable in an expression that sql query a table access 2007.

I've read some articles about using the IS NULL and IS NOT NULL and made ​​a few unsuccessful attempts.

I tried and did not work well:

sql = "SELECT * FROM Registration"
sql = sql & "WHERE Field1 = '" & variable1 & "' AND Field2 = '" & variable2 & "' AND IS NOT NULL Field3 = '" & variable3 & "'"

In my example, the variable3 is empty.

Anyone know how to say the words correctly write?

Sorry if I wrote it wrong.

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