SQL TOP ការប្រើ

Course Recommendation:

TOP Clause

The TOP clause is very useful for large tables with thousands of records.

Note:Not all database systems support the TOP clause.

SQL Server Syntax:

SELECT TOP number|percent column_name(s)
FROM table_name

SQL SELECT TOP is equivalent in MySQL and Oracle

MySQL Syntax

SELECT column_name(s)
FROM table_name
LIMIT number

Example

SELECT *
FROM Persons
LIMIT 5

Oracle Syntax

SELECT column_name(s)
FROM table_name
WHERE ROWNUM <= number

Example

SELECT *
FROM Persons
WHERE ROWNUM <= 5

Original Table (used in examples):

Persons Table:

Id LastName FirstName Address City
1 Adams John Oxford Street London
2 Bush George Fifth Avenue New York
3 Carter Thomas Changan Street Beijing
4 Obama Barack Pennsylvania Avenue Washington

SQL TOP Example

Now, we want to select the first two records from the above 'Persons' table.

We can use the following SELECT statement:

SELECT TOP 2 * FROM Persons

Result:

Id LastName FirstName Address City
1 Adams John Oxford Street London
2 Bush George Fifth Avenue New York

SQL TOP PERCENT Example

Now, we want to select 50% of the records from the above 'Persons' table.

We can use the following SELECT statement:

SELECT TOP 50 PERCENT * FROM Persons

Result:

Id LastName FirstName Address City
1 Adams John Oxford Street London
2 Bush George Fifth Avenue New York