What does all mean in sql

ALL in SQL indicates that the query will return all matching rows, including duplicate rows. Use ALL to disable deduplication and allow multiple rows to have the same value: Add ALL to the SELECT statement: SELECT ALL column_name(s) FROM table_name WHERE condition. Use ALL when you need to include duplicate values, count rows or aggregate values, or disable deduplication in a subquery.

ALL in SQL

What is ALL?

ALL is a keyword in SQL that indicates that the query should return all matching rows in the table, regardless of duplicates. In other words, ALL disables deduplication, allowing queries to return multiple rows with the same value.

How to use ALL?

To use ALL, add it to the SELECT statement as follows:

<code class="sql">SELECT ALL column_name(s)
FROM table_name
WHERE condition;</code>

Example 1

Assume there is a name The table "customers", which contains customer data:

<code class="sql">| customer_id | customer_name |
|-------------|---------------|
| 1            | John Doe       |
| 2            | Jane Doe       |
| 3            | John Doe       |</code>

If ALL is not used, the SELECT statement will only return a unique result:

<code class="sql">SELECT customer_name
FROM customers
WHERE customer_id = 1;</code>

Output:

<code>John Doe</code>

However, If ALL is used, the query returns all matching rows, including duplicate values:

<code class="sql">SELECT ALL customer_name
FROM customers
WHERE customer_id = 1;</code>

Output:

<code>John Doe
John Doe</code>

When to use ALL?

ALL is usually used in the following situations:

• Needs to return all matching rows, including duplicate values.
• Need to count rows or aggregate values ​​(such as SUM and COUNT) where duplicate values ​​are important.
• In subqueries, deduplication needs to be disabled to get correct results.

Note:

You need to pay attention to the following points when using ALL:

• ALL will increase the execution time and resources of the query consumption.
• ALL will cause the result set to contain a large amount of duplicate data, thus affecting the efficiency of subsequent processing.
• In most cases, it is not recommended to use ALL unless repeated values ​​are explicitly required.

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