Fortunately, I have written many complex SQL queries in my career. But the question is not about which one was the most complex query, but we should focus on how to write complex SQL queries with an ease.

Well, there is no most-complex SQL queries, any SQL query can turn out to be a complex task if you are not proficient in the same.

To succeed, you should be confident enough and must have prior experience in dealing with SQL syntax

When you have got complex SQL query including many joints across various tables, many different nested or hidden conditions, then writing that query without any hassle becomes a little intimidating. So, if you have prior experience in writing a quality SQL query, you might write complex queries without a hitch.

Here is something that can help to write a complex SQL query with ease;

• Know what kind of end result you are working for- Having an idea about how many records you are expecting to retrieve and which tables should be used for the desired results will be a great help.
• A proper understanding of Table relationships- Only if you know different fields in tables and the relationship between these tables, fields, etc., you can end-up in writing a complicated SQL query.
• Split the query into different parts- This will make the query easy manageable. Start with two simple parts, then include others. Test simpler queries first at every stage and then proceed further.

Keep the SQL query simple, easy to read and carry on carefully. With this, you can efficiently write the complex SQL query without a hitch.