SQL Queries Execution Order

👉 The order of SQL queries execution can be demystified by understanding the logical flow of the SQL statement processing.

👉 Generally, the execution follows a specific order:

• FROM clause: This specifies the tables from which the data will be retrieved.
• WHERE clause: This filters the rows based on specified conditions.
• GROUP BY clause: This groups the rows based on specified columns.
• HAVING clause: This filters the grouped rows based on specified conditions.
• SELECT clause: This selects the columns to be displayed in the result set.
• ORDER BY clause: This sorts the result set based on specified columns.
• LIMIT clause: This limits the number of rows returned in the result set.

👉 It’s important to note that not all clauses are required in every SQL statement. The execution order may vary depending on the specific query and the database management system being used. Understanding the order of execution can help optimize queries and improve performance.

👉 Here are some SQL query optimization techniques to get precise data from the database.

1️⃣ SELECT fields, rather than using SELECT: Use the SELECT statement optimally, instead of always fetching all data from the table. Fetch only the necessary data from the table, thereby avoiding the costs of transferring unwanted data and processing it.

2️⃣ Avoid DISTINCT in SELECT query : SELECT DISTINCT is a simple way of removing duplicates from a database. SELECT DISTINCT works to generate distinct outcomes by using the GROUP BY clause, which groups all the fields in the query. However, a large amount of processing power is required to do this. So, avoid DISTINCT in SELECT queries.

3️⃣ Use WHERE instead of HAVING: The HAVING clause filters the rows after all the rows are selected. It works just like a filter. Do not apply the HAVING clause for any other purpose. HAVING statements are determined in the SQL operating order after WHERE statements. Therefore, it is quicker to execute the WHERE query.

4️⃣ To check the existence of records, use EXISTS() rather than COUNT(): Both EXISTS() and COUNT() methods can be used to check the existence of a record entry in the table. The EXISTS() method is more effective as it exits processing as soon as it finds the first entry of the record in the table. The COUNT() method would scan the entire table to return the number of records in the table that match the provided constraint.

5️⃣ Avoid ORDER BY, GROUP BY, and DISTINCT: Use ORDER BY, GROUP BY, and DISTINCT only when necessary. SQL creates work tables and puts the data there. It then organizes the data in the work table based on the query and then returns the results.

6️⃣ Use conditional WHERE clause: Conditional WHERE clauses are used for subsetting

7️⃣ Create JOINs with INNER JOIN (not WHERE): The SQL INNER JOIN statement returns all matching rows from joined tables, while the WHERE clause filters the resulting rows based on the specified condition. Retrieving data from multiple tables based on the WHERE keyword condition is called NON-ANSI JOINs while INNER JOIN belongs to ANSI JOINs.

It does not matter for SQL Server how you write the query – using ANSI or NON-ANSI joins – it’s just much easier to understand and analyze queries written using ANSI joins. You can clearly see where the JOIN conditions and the WHERE filters are, whether you missed any JOIN or filter predicates, whether you joined the required tables, etc.

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