Id:182-15-2189.

A complex SQL query is a combination of many joins and subqueries. SQL can be as complicated as you want. 

Sql queries can be complicated with additional clause, sub queries, UNION, and so on. There are also stored procedures and triggers in most implimentation of sql.

Sql queries can start from a simple tasks bt grow very complicated with nested queries, common table expressions. So, maybe sql can be simple bt can be very complicated too!