Example:

Consider the CUSTOMERS table having the following records:

| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |

Following is an example on Oracle server, which would fetch top 3 records from CUSTOMERS table:

SQL> SELECT * FROM CUSTOMERS
WHERE ROWNUM <= 3;

This would produce the following result:

| ID | NAME    | AGE | ADDRESS   | SALARY  |
+----+---------+-----+-----------+---------+
|  1 | Ramesh  |  32 | Ahmedabad | 2000.00 |
|  2 | Khilan  |  25 | Delhi     | 1500.00 |
|  3 | kaushik |  23 | Kota      | 2000.00 |

Read more: SQL - TOP, LIMIT or ROWNUM Clause

Operators in The WHERE Clause

The following operators can be used in the WHERE clause:

Operator	Description
=	Equal
<>	Not equal. Note: In some versions of SQL this operator may be written as !=
>	Greater than
<	Less than
>=	Greater than or equal
<=	Less than or equal
BETWEEN	Between an inclusive range
LIKE	Search for a pattern
IN	To specify multiple possible values for a column

Read more: SQL WHERE Clause

The HAVING Clause

The HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions.

SQL HAVING Syntax

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value;

SQL HAVING Example

|OrderID |EmployeeID|ShipperID  |
+--------+----------+-----------+
|10248	 | 5        |	3       |
+--------+----------+-----------+
|10249	 | 6        |	1       |
+--------+----------+-----------+
|10250	 | 4        |	2       |

And a selection from the "Employees" table:

|EmployeeID |LastName |
+-----------+---------+
|1          |Davolio  |
+-----------+---------+
|2          |Fuller   |
+-----------+---------+
|3          |Leverling|

Now we want to find if any of the employees has registered more than 10 orders.
We use the following SQL statement:

SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders FROM (Orders
INNER JOIN Employees
ON Orders.EmployeeID=Employees.EmployeeID)
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 10;

Read more: SQL HAVING Clause

SQL HAVING Example

|OrderID |EmployeeID|ShipperID  |
+--------+----------+-----------+
|10248	 | 5        |	3       |
+--------+----------+-----------+
|10249	 | 6        |	1       |
+--------+----------+-----------+
|10250	 | 4        |	2       |

And a selection from the "Employees" table:

|EmployeeID |LastName |
+-----------+---------+
|1          |Davolio  |
+-----------+---------+
|2          |Fuller   |
+-----------+---------+
|3          |Leverling|

Now we want to find if the employees "Davolio" or "Fuller" have registered more than 25 orders.
We add an ordinary WHERE clause to the SQL statement:

SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders
INNER JOIN Employees
ON Orders.EmployeeID=Employees.EmployeeID
WHERE LastName='Davolio' OR LastName='Fuller'
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 25;

SQL DROP INDEX, DROP TABLE, and DROP DATABASE

Indexes, tables, and databases can easily be deleted/removed with the DROP statement.
Read more: SQL DROP

The DROP INDEX Statement

The DROP INDEX statement is used to delete an index in a table.
DROP INDEX Syntax for MS Access:

DROP INDEX index_name ON table_name

DROP INDEX Syntax for MS SQL Server:

DROP INDEX table_name.index_name

DROP INDEX Syntax for DB2/Oracle:

DROP INDEX index_name

DROP INDEX Syntax for MySQL:

ALTER TABLE table_name DROP INDEX index_name

Read more: SQL DROP

The DROP TABLE Statement

The DROP TABLE statement is used to delete a table.

DROP TABLE table_name

Read more: SQL DROP

The DROP DATABASE Statement

The DROP DATABASE statement is used to delete a database.

DROP DATABASE database_name

Read more: SQL DROP

The TRUNCATE TABLE Statement

What if we only want to delete the data inside the table, and not the table itself?
Then, use the TRUNCATE TABLE statement:

TRUNCATE TABLE table_name

Read more: SQL DROP