Usage of all in sql

ALL operator is used to combine all rows of multiple query results, as opposed to DISTINCT which only returns unique rows. It is used to return all rows, ignore duplicates, and handle NULL values. The difference with DISTINCT is that ALL returns all rows, including duplicate rows, while DISTINCT returns only unique rows.

Usage of ALL operator in SQL

Introduction

The ALL operator is used in SQL to combine all rows from multiple query results. It is the opposite of the DISTINCT operator, which returns only unique rows.

Syntax

<code>SELECT column_list
FROM table1
[INNER | LEFT | RIGHT | FULL] JOIN table2
ON join_condition
ALL</code>

Usage

The ALL operator is used in the following scenarios:

• Return all rows: When you need to merge all rows in multiple query results, ALL can ensure that no rows are discarded.
• Ignore duplicates: Unlike DISTINCT, ALL returns all rows in multiple query results, even if they are duplicates.
• Handling with NULL values: The ALL operator treats NULL values ​​as any other value, which means that even if a NULL value exists in a query result, ALL will still return the row.

Example

The following example demonstrates how to use the ALL operator:

<code>SELECT *
FROM customers
ALL
SELECT *
FROM orders;</code>

This query will return all rows, including duplicate rows.

Differences from DISTINCT

The DISTINCT operator returns only unique rows, while the ALL operator returns all rows, including duplicate rows. The following example illustrates the difference:

<code>SELECT DISTINCT customer_id
FROM customers;

SELECT customer_id
FROM customers
ALL
SELECT customer_id
FROM orders;</code>

The first query returns only unique customer IDs from the customers table, while the second query returns all customer IDs, including duplicate customer IDs.

Notes

• The ALL operator may result in large result sets, especially when the query involves large tables.
• When using the ALL operator, you must ensure that the join conditions are correct to avoid unnecessary Cartesian products.

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