Syntax

    SELECT table1.a, table1.b FROM table1 WHERE EXISTS 
        (select * from table2 where table1.a = table2.a)

The sub-query (select * from table2 where table1.a = table2.a) is solved for each row in table1.
When the query is processed and is stepping through each record in the outer query (table1), the data for table1.a is used in the inner sub-query.
This means that if there are 1,000 rows in the outer query, the inner sub-query will be executed 1,000 times.
Care should be taken to only use correlated sub-query when the tables are small.
If the sub-query returns 1 or more records, the EXISTS sub-query is true and the record is placed in the return set.
If the statement NOT EXISTS was used instead, the data from the outer query row would only be included in the return set if the sub-query returned no records.
Using the Test gsSQL Google Sheet, you can use the existing data (you don’t need to enter data for the tables) and try out:

Find open space on the first page and type into the cell:

 =gsSQL("select * from customers where exists (SELECT * FROM booksales WHERE booksales.customer_id = customers.id) and email like '%gmail.com'")

You should see results like this (What it means: Show me customers that had a sale and thier email ends with “gmail.com”).

CUSTOMERS.ID	CUSTOMERS.NAME	CUSTOMERS.ADDRESS	CUSTOMERS.CITY	CUSTOMERS.PHONE	CUSTOMERS.EMAIL
C1	Numereo Uno	101 One Way	One Point City	9051112111	bigOne@gmail.com
C2	Dewy Tuesdays	202 Second St.	Second City	4162022222	twoguys@gmail.com

Try the NOT EXISTS sql like this:

  select * from customers where not exists 
      (SELECT * FROM booksales WHERE booksales.customer_id = customers.id)

Add to your sheet.

Copy the contents of gsSQL Custom Function to your copy buffer.
On your test sheet, create a new Apps Script (under Extensions) and paste in the contents to a new file.
Once you save, the custom function gsSQL() will be available from any cell on your sheet.
- Just start typing =gsSQL(“”) (and insert your own SELECT statement).