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What is Complex SQL Queries? Explain Complex SQL Queries with Examples

Debugging Complex SQL Queries – The General Process

Debugging Complex SQL Queries

Check on syntax and logic:

The first basic step is investigating the SQL code. The different SQL statements like an update, insert, or delete statements can be nested together. You need to work on the testing logic and separate the level of nesting too. Now substitute representative values into correlated subqueries for the automatic execution. These subsequent debugging steps are able to handle only one query at a time.

Check on SQL joins:

Once you have resolved issues related to syntax or flow, the next focus is joining logic. It is easy to run afoul of joins so we have to make sure that we are not losing records accidentally. An inner join may result in unintended loss of records when not used properly. In a few cases, an inner join is also restarted as the outer join.

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Check on Predicates:

Next, we have to restore the “where predicates” and run the query again. In case, the query fails to run, there is clearly something wrong with predicates. Read the code carefully that will help you to find errors certainly. If you are still not able to spot errors then step back and restore a few predicates at one time. Eventually, when the focus gets narrowed, spotting errors becomes more obvious.

Read: SQL Intersect Operator With Example

Check on attributes:

If joins and predicates both are correct, the next step is to focus on attributes. We introduce multiple attributes together and you have to make sure all attributes ensure the right results.

Check on nesting queries:

In the final step, put the nested queries together from the initial SQL statement. We should test the SQL statements thoroughly to make sure that it runs correctly.

It is true that developers get frustrated when query execution fails multiple times. The straight forward approach pulls the statements together and helps to find errors. In practice, you will observe that debugging complex SQL queries is much easier than your expectations. The SQL code is easy to break down into a chunk of pieces that can be tested individually and nested together once the testing is done.

Read: Top 50 Datastage Interview Questions and Answers


Offloading of application logic to SQL queries for the developers is quite common as it saves time, effort, and reduces errors by substituting SQL for the programming code. It leads to complex SQL queries that should be debugged properly. It is common to write a SQL query that fails to execute initially. There can be multiple reasons for execution failure like misspelling a name or keyword, omitting keyword, inconsistent data flow, illegal function usage, database error messages, etc that pinpoints that something is wrong with the SQL query.

We usually inspect queries but not able to spot errors, so it is necessary to debug complex SQL queries with the right process and technique. Let us discuss the general process first how to debug complex queries in a few simple steps.

Things to Consider when Writing Complex SQL Queries in 2020

When you are given to write a complex SQL query that involves joins across different tables, multiple nested conditions then it may sound intimidating, especially when you are not so confident or experienced with SQL syntax. Here, we will learn how to write a complex SQL query and breaking it down into simple steps and building it back from there.

Complex SQL Queries

Let us discuss each of the steps one by one in brief below.

Focus on the final output

Start with the final outcome in the mind and write down the desired result set from the SQL query. Evaluate how many records you want to retrieve and which tables can be used to derive the needed output.

Understand the database relationships

The second step in writing a complex SQL query is a good understanding of database relationships among different fields. Figure out whether it is one to one or one to many relationships. AN ER (Entity Relationship) diagram is considered most useful here to understand the relationships quickly. Make sure you know about fields that are needed for joins.

Read: Microsoft Power BI Certification Exam- 2024 Updated Guidelines

Break the query into manageable parts

Keep in mind that you don’t have to write the complete SQL query at once. Start by determining joins that are needed for the table and introduce other tables accordingly. Remember that a complex SQL query is a collection of multiple simple SQL queries. Test the queries one by one at each stage to make sure that we are getting the same output as needed.

Once a query is created, it should be tested again and again as soon as a new table is added. This continuous testing will make sure that you are still getting the same results that are required.

Aggregate the result sets

This is the time to aggregate the different result sets with Group By statement. You may also use multiple aggregate functions on the basis of requirements like COUNT, MAX, SUM, MIN, SUM, or AVG, etc. Also, if the order of the result set matters then put them in proper order with Order by the statement.

Read: The Database as Part of the System Architecture

Make the query easy to read

Add comments for the SQL queries where needed. Don’t put unnecessary comments otherwise, it will make the query lengthy and boring. Keep in mind that longer the SQL statement, more bandwidth is required for the statement execution. Try to use standard conventions that are used frequently by most of the developers.

Other facts to consider

For a complex SQL query, it is common to use a stored procedure for the query however there are a number of other facts to consider when writing complex SQL queries.

  • Complex SQL queries are usually not portable if you want to use another DBMS.
  • There are notoriously difficult to test when there is minimal debugging.
  • They decrease the load on the application server but increases the load on the web server.

You may use simple SQL statements then loop them together using more SQL statements when required. The experienced database developer will never recommend this route as it is tough and messy. The best idea is to practice the SQL code and become more efficient in writing complex SQL queries with the right technique or approach. 

Join our self-learning courses today and learn all about writing queries in SQL using the best programming practices.

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Why should you avoid writing complex SQL queries?

A complex SQL query is a combination of many joins and subqueries, soon you will realize that long queries are the source of the bug as well. So, the question here is whether you should hunt for the original creator to debug the query or you should re-engineer the query yourself with a deep understanding of hidden complexities.

The biggest benefit of complex SQL queries is to fetch data from multiple data sources but it may create problems when overused by developers. In the case of complex SQL queries, developers have to define relationships between different pieces of data, join tables, group the result set, order the result set, and so on.

When you are interested in modifying the database schema, it usually involves re-engineering a query. Sometimes, developers prefer not to change the data model in fear of recreating a large number of SQL queries and arrange them in a specified order.

Read: Learn SQL Union All Query Operators with Examples

When there is so much headache associated with the complex SQL query design, most of the developers are interested in using alternatives if any.

Alternatives to Complex SQL Queries

There are two popular alternatives to complex SQL queries as given below.

  1. Either use plenty of small SQL queries, OR
  2. You may use query builders

The first alternative is pretty simple. Instead of hitting the database with one large query that contains everything at once, try to break down the query into smaller manageable units. Write multiple subqueries to grab different data sections, process them individually instead of using aggregate functions all the time. For novice programmers, it is the most powerful approach that helps to understand the complex query thoroughly without jumping directly to the most performant solution.

Read: How to Restore a Database Backup from SQL Server Management Studio

Don’t forget to consider the trade-offs inefficiency when you want to use small queries. Small queries usually have less tendency to manage responsibility, and they can be applied freely in different situations. According to experienced DBAs, small queries with a selected set of responsibilities are generally good in their job, easy to debug, and there are fewer syntax errors too.

The second alternative involves using query builders. It is just the best idea of breaking down the large query into small manageable parts that describe different aspects of a query. It will give you a set of multiple queries with shared concepts. You can work on a particular set individually and try to test it thoroughly for logic or syntax error so that it can be executed successfully later.

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Final Thoughts

Complex SQL queries have their special place in the software development process but there are some issues associated with every high-performing too and the same is the concept with complex SQL queries too. Developers recommend breaking down the complex queries into manageable bits. Small queries and always flexible and easy to handle in evolving different pieces of the software. They are also considered good in terms of performance. 

To know more about advanced SQL concepts and how to write complex SQL queries, sign up for the SQL certification program at JanBask Training and get hands-on expertise on different database concepts or tools.

Read: What is Update Query in SQL? How to Update (Column Name, Table, Statement, Values)

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