Definition - What does Complex SQL mean?
Complex SQL is the use of SQL queries which go beyond the standard SQL of using the SELECT and WHERE commands. Complex SQL often involves using complex joins and sub-queries, where queries are nested in WHERE clauses. Complex queries frequently involve heavy use of AND and OR clauses. These queries make it possible for perform more accurate searches of a database.
Techopedia explains Complex SQL
Complex SQL queries go beyond the standard SQL query commands such as SELECT. These queries can use many embedded clauses to weed out irrelevant information and join different tables. One frequently used SQL technique is the use of sub-queries embedded in a query within parentheses. Another technique is the self-join, which treats one table as two different tables, allowing it to match multiple values in a row. While these queries allow for very specific and flexible searches, the disadvantage is that they can be hard to understand.