Treat empty variable - Excel VBA

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:

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In my example, the variable3 is empty.

Anyone know how to say the words correctly write?

Sorry if I wrote it wrong.

At

Do you actually need variable3 in the SQL if it's empty?

If you don't you could try something like this.

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