A complex SQL query is a combination of many joins and sub queries.The compositional form Mark's it easy to write complex queries & computations over the database.Complex SQL is the use of SQL queries which go beyond the standard SQL of using the SELECT & WHERE commands .Complex SQL often involves using nested in WHERE clauses.Thats a SQL can be complicated. 

SQL queries can extend to any complexity with additional clauses, subqueries, UNION, and so on. You can (in theory) make a query that is many pages long.