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SQL Tutorial: How To Write Better Queries

Learn about anti-patterns, execution plans, time complexity, query tuning, and optimization in SQL.
Updated Dec 2022  · 35 min read

Structured Query Language (SQL) is an indispensable skill in the data science industry and generally speaking, learning this skill is relatively straightforward. However, most forget that SQL isn’t just about writing queries, which is just the first step down the road. Ensuring that queries are performant or that they fit the context that you’re working in is a whole other thing.

That’s why this SQL tutorial will provide you with a small peek at some steps that you can go through to evaluate your query:

Writing SQL Queries FAQs

How can I improve the performance of my SQL queries?

There are several ways you can make your SQL queries perform better: 

  • Use appropriate indexes to speed up queries that filter or sort large datasets.
  • Avoid using functions on columns in the WHERE clause, as these can prevent the use of indexes.
  • Use EXPLAIN to understand the execution plan of your queries and identify potential performance bottlenecks.
  • Use LIMIT and OFFSET appropriately to avoid retrieving more data than you need.
  • Use subqueries and derived tables sparingly, as they can be expensive to execute.

How can I make my SQL queries more readable?

These tips can help you write SQL queries that are more readable: 

  • Use meaningful and descriptive names for tables, columns, and aliases.
  • Use white space and indentation to make the structure of your queries more clear.
  • Use comments to document your queries and explain your reasoning.
  • Use uppercase for SQL keywords and lowercase for everything else to improve readability.

How can I avoid common SQL query mistakes?

To avoid some common mistakes when writing SQL queries: 

  • Make sure you use the correct comparison operator (e.g., = instead of ==).
  • Use single quotes around string literals, not double quotes.
  • Be careful with NULL values – they don't behave like other values in comparisons.
  • Use AS to give columns and tables aliases rather than renaming them directly.
  • Use parentheses to group and order your clauses correctly

How can I write more complex SQL queries?

Here are some ways to add complexity to your SQL queries:

  • Use CASE statements to add conditional logic to your queries.
  • Use UNION and UNION ALL to combine the results of multiple SELECT statements.
  • Use subqueries to perform additional queries within your main query.
  • Use window functions to perform calculations across rows of a result set.
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