Understanding SQL Query Order of Operations

Programming languages like C, C++, Python, and Java possess the order of precedence and associativity for operators. There is a pre-determined order that determines which operator will execute sooner in implementation. A parallel concept is available in SQL, where the order of operation is pre-determined. When we write a query to crack problems, it contains diverse clauses like FROM, WHERE, GROUP BY, etc. These clauses will perpetrated in a certain order, which is known as the order of operation in SQL. Let’s unearth the concept with illustrations.

Why Order of Operations Matters?

SQL is a declarative language, which implies that you can write logic and get or manipulate the data from the database utilizing a query. We are conducting queries on databases and tables. This query should be logically correct and legible.

1. The order of operation will allow us to write this query in a systematic mode using SQL.
2. The second reason to obey the order of operation while writing a query is readability and maintaining logic.
3. Optimization is feasible due to the order of operation in SQL. The developer can easily visualize the logic and optimize the solution accordingly.
4. Another benefit of using the order of operation in SQL is the lower error probability. As we have discussed, the logic is clear due to the order of operation the developer knows the outcome.

When we obey the order of operation, the logic is straightforward and perceptible to developers. It will relieve the complexity of the problem. There are various techniques and modes to decode the same problem. Order of operation will aid in deciding which one is better. Therefore, we are only required to obey the order for correct results.

Order of Operations in SQL

In simple words, the order of operation is the appropriate way of managing different clauses, keywords, joins, subqueries, etc. This order of operation is straightforward to imagine in a table layout. Let’s see the table and comprehend the concept. 

In the table, the FORM clause on No. 1, then JOIN, WHERE, and other clauses. In the bottom line, we can see the Expression evaluation code. While writing the query, we need to follow this order of operation. There is a specific reason for this order, and it is related to their function. We will shortly discuss the function of every clause and some examples to prove the importance of the order of operation.

1. FROM Clause

The FROM clause will enable us to appoint the table or database we are using to execute a query. That is why this clause comes first. If there is more than one table, then we can join them in this step. The rows are also selected in this operation.  

Syntax of FROM Clause:

SELECT column_name1, column_name2
FROM table_name;
Code language: SQL (Structured Query Language) (sql)

This is a basic syntax for the FROM Clause.

2. JOIN Clause

The JOIN Clause will enable to join the tables obtainable in the database. Sometimes, we require more than one table to get the expected outcome. The JOIN Clause will be applied after the FROM Clause. 

Syntax of JOIN Clause:

SELECT column1, column2
FROM table1
JOIN table2 ON table1.column1 = table2.column2;
Code language: SQL (Structured Query Language) (sql)

You can use this basic syntax in your query.

3. WHERE Clause

The clause that extracts the data from the table using conditions. The prerequisites are applied to rows. The WHERE clause is very important because it operates like a filter at this phase. 

Syntax of WHERE Clause:

SELECT column1, column2
FROM table_name1
WHERE condition;
Code language: SQL (Structured Query Language) (sql)

Inside the WHERE clause, we can employ additional functions, clauses, and operators to impose the conditions. 

4. GROUP BY Clause

The GROUP BY clause will allow you to compute the aggregates of the data present in the table. GROUP BY clause is used after the WHERE clause when data is filtered according to the basic requirements. 

Syntax of GROUP BY Clause:

SELECT column1, column2
FROM table_name
GROUP BY grouping_column1, grouping_column2;
Code language: SQL (Structured Query Language) (sql)

This clause helps to get the summary of the data, column, table, etc, from the database.

5. HAVING Clause

The HAVING Clause operates similarly to the WHERE clause. The prerequisites can be applied using this clause. The data is sorted using the HAVING Clause. The only distinction between the WHERE Clause and the HAVING Clause is the data on which they are applied. The WHERE clause is conducted on single data elements, while the HAVING clause is executed on the grouped data points.

Syntax of HAVING Clause:

SELECT column1, column2, column3
FROM table_name1
GROUP BY grouping_column1, grouping_column2
HAVING condition1;
Code language: SQL (Structured Query Language) (sql)

 As you can see, the GROUP BY clause will calculate the aggregates and the HAVING Clause will apply to it. 

6. SELECT Clause

If you see apparently, the SELECT Clause will help you to print the table. It just represents the outcome after applying different clauses, functions, and operators.

Syntax of SELECT Clause:

SELECT column1, column2
FROM table1;
Code language: SQL (Structured Query Language) (sql)

It is mostly used at the beginning of the query.

7. ORDER BY Clause

The ORDER BY clause will sort out the final data after the application of conditions on aggregated data elements. The OREDER BY clause will applied in ascending and descending order on the data. 

Syntax of ORDER BY Clause:

SELECT column1, column2
FROM table_name1
ORDER BY column1 [ASC|DESC], column2 [ASC|DESC];
Code language: SQL (Structured Query Language) (sql)

You can use this query on your data, tables, and columns. 

8. LIMIT/OFFSET Clause

This LIMIT/OFFSET clause will help to limit the number of rows displayed in the outcome. LIMIT/OFFSET clause avoids confusion while displaying the outcome. The size of the result is managed using this clause. 

Syntax of LIMIT/OFFSET Clause:

SELECT column1, column2
FROM table_name1
LIMIT row_count OFFSET offset_value;
Code language: SQL (Structured Query Language) (sql)

You only need to set the row count, and offset count and it will display the correct result.

9. Expression Evaluation

If we are using any custom function in the query, then this is the phase where it is calculated. Generally, the SELECT clause is used here. The calculation, and function evaluation are done at this stage.

Syntax of Expression Evaluation:

SELECT
  expre1,
  expre2 AS alias,

FROM
  table_name1;
Code language: SQL (Structured Query Language) (sql)

These are some basic orders of operations available in SQL language. If you follow these while writing any query, you will never mess up your code. The query will throw a syntax error if we do not follow the order of operation.

Summary

In this article, we have discussed the basic concept of SQL language i.e., order of operation. The order of operation is very straightforward and important to write a correct query. This order of operation always helps to understand the logic behind the query. There are basic 7 clauses, which are mainly in order of operation. We have written some extra clauses and code syntax, which is part of every query. This will help you to understand the structure and nature of SQL queries. Hope this writing will help you.

Reference

https://stackoverflow.com/questions/879893/sql-order-of-operations