SQL TOP statement: limit the number of returned data

The TOP clause in SQL Server is used to specify the number of records to return from a select query. The TOP clause can be very useful on large tables with thousands of records, where returning a large number of records can impact performance.

Syntax:

SELECT TOP number|percent column_name(s)
FROM table_name
WHERE condition;

The number|percent indicates that you can either specify a specific number of records to return, or a percentage of records to return.

Example 1: Return only the first 5 records.

SELECT TOP 5 *
FROM Employees;

Example 2: Return the top 10 percent of records.

SELECT TOP 10 PERCENT *
FROM Employees;

You can also use the TOP clause with the ORDER BY clause to return the top records based on a sort.

Example 3: Return the 3 employees with the highest salaries.

SELECT TOP 3 *
FROM Employees
ORDER BY Salary DESC;

Note: In other SQL systems, such as MySQL and PostgreSQL, the LIMIT clause provides similar functionality to the TOP clause.

Please note that the usage of TOP without an ORDER BY clause might not give you consistent results, as there's no guaranteed order of rows in a SQL table without an ORDER BY clause. If you want a consistent result, always use TOP with an ORDER BY clause.

As always, the exact syntax and capabilities of the TOP clause can vary between different SQL dialects, so it's important to consult the documentation for the SQL variant you're using.

How to Use TOP in SQL to Restrict Results:
- Description: The TOP keyword is used in SQL Server to limit the number of rows returned in a result set.
- Code Example:
```
SELECT TOP 5 column1, column2
FROM example_table;
```
SQL SELECT TOP N Rows:
- Description: The TOP clause can be used with a numerical value (N) to specify the maximum number of rows to be returned.
- Code Example:
```
SELECT TOP 10 column1, column2
FROM example_table;
```

Examples of Limiting Rows in SQL Using TOP:

Code Example 1:
```
SELECT TOP 5 * FROM example_table;
```

Code Example 2:

SELECT TOP 3 column1, column2
FROM example_table
WHERE condition;

Using TOP to Fetch a Specific Number of Rows in SQL:
- Description: Specify the number of rows to be retrieved by providing a numerical value after the TOP keyword.
- Code Example:
```
SELECT TOP 8 column1, column2
FROM example_table
ORDER BY column1;
```
MySQL LIMIT vs SQL Server TOP:
- Description: In MySQL, the LIMIT keyword is used to restrict the number of rows, while SQL Server uses TOP.
- Code Example (MySQL):
```
SELECT column1, column2
FROM example_table
LIMIT 5;
```
PostgreSQL Equivalent of SQL Server TOP:
- Description: In PostgreSQL, the LIMIT keyword is used similar to MySQL to restrict the number of rows.
- Code Example (PostgreSQL):
```
SELECT column1, column2
FROM example_table
LIMIT 7;
```
Fetching Top Records in Oracle SQL:
- Description: In Oracle SQL, you can use the ROWNUM or FETCH FIRST clause to limit rows.
- Code Example (ROWNUM):
```
SELECT column1, column2
FROM example_table
WHERE ROWNUM <= 6;
```
- Code Example (FETCH FIRST):
```
SELECT column1, column2
FROM example_table
ORDER BY column1
FETCH FIRST 4 ROWS ONLY;
```