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Except - (PostgreSQL Advance)

In PostgreSQL, EXCEPT is an operator that returns all distinct rows from the first query that are not in the result of the second query. In other words, it returns the difference between two result sets. In this tutorial, we'll discuss the syntax, example, output, explanation, use, important points, and summary of using EXCEPT in PostgreSQL.

Syntax

The basic syntax for using EXCEPT in PostgreSQL is as follows:

SELECT column1, column2, ...
FROM table1
EXCEPT
SELECT column1, column2, ...
FROM table2;

The EXCEPT operator takes two queries and returns the unique rows in the first query that are not in the second query.

Example

Let's take a look at an example of using EXCEPT in PostgreSQL.

SELECT * FROM customers
EXCEPT
SELECT * FROM orders;

In this example, the EXCEPT operator takes the result set of the first query, which selects all rows from the customers table, and returns all of the distinct rows that are not in the result set of the second query, which selects all rows from the orders table.

Explanation

In the above example, we selected all rows from the customers table and returned distinct rows that are not in the orders table. This means that we returned a list of all customers who have not placed an order. In other words, we returned the difference between the two tables.

Use

The EXCEPT operator is often used for comparing two result sets. It can be used to find the differences between two tables or to return unique values in one table that are not in another table.

Important Points

  • The EXCEPT operator returns distinct rows, so duplicate rows in the first query that are also in the second query will only appear once in the result set.
  • The columns in the SELECT clause of both queries must be of the same data type and be in the same order.
  • The result set of the EXCEPT operator is read-only and cannot be modified.
  • Both queries must be compatible with each other, meaning they must have the same number of columns and their respective columns must have the same data type.

Summary

In this tutorial, we discussed the EXCEPT operator in PostgreSQL. We covered the syntax, example, output, explanation, use, important points, and summary of using EXCEPT in PostgreSQL. With this knowledge, you can now use the EXCEPT operator to find the differences between two tables or to return unique values that are not in another table.

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