TOP vs. LIMIT in SQL: When Should I Use Each Keyword?

Understanding the Differences Between TOP and LIMIT Keywords in SQL

In the world of Structured Query Language (SQL), the choice between the TOP and LIMIT keywords can impact the execution and result set of your queries, particularly when retrieving the first few rows of a table.

TOP Keyword (SQL Server)

The TOP keyword is primarily used in Microsoft SQL Server and returns the specified number of top rows from a table based on a predefined order. The order is determined by the sort order of the data, which can be specified using an ORDER BY clause. For example, the query:

SELECT TOP 2 * FROM Persons
ORDER BY LastName;

will retrieve the top 2 rows from the Persons table, ordered in ascending order of the LastName column.

LIMIT Keyword (MySQL, PostgreSQL)

On the other hand, the LIMIT keyword is commonly used in MySQL and PostgreSQL. It returns the specified number of rows starting from the beginning of the dataset. The order of the rows is not affected by the LIMIT clause. For example, the query:

SELECT * FROM Persons
LIMIT 2;

will retrieve the first 2 rows from the Persons table regardless of their order.

Usage Scenarios

The choice between TOP and LIMIT depends on the database being used and the specific requirements of the query:

• Use TOP when you need to retrieve the top rows based on a specific ordering, as in the case of retrieving the 5 highest-paid employees from a table.
• Use LIMIT when you want to retrieve a specific number of rows from the beginning of the dataset, such as displaying the first 10 results of a search.

Database Behavior

The database executes both TOP and LIMIT queries in different ways:

• TOP: The database evaluates the WHERE clause first, filters the rows based on the specified conditions, and then applies the TOP keyword to return the top rows.
• LIMIT: The database fetches all rows that meet the WHERE clause criteria and then returns the specified number of rows from the beginning.

In summary, TOP and LIMIT are database-specific keywords used to retrieve a limited number of rows from a table. TOP orders the rows by a predefined sort order, while LIMIT retrieves rows from the beginning of the dataset. Choose the appropriate keyword based on the database being used and the desired query behavior.

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