In SQL Server, the
WHERE operator is used in a SELECT statement to filter data based on a specified condition or set of conditions. The WHERE operator allows you to retrieve only the rows that meet a specific criteria, rather than returning all the rows from a table.
The syntax for the
WHERE operator is as follows:
SELECT column1, column2, ...
The condition can be a simple comparison using one of the comparison operators (=, <>, <, >, <=, >=), or it can be a more complex expression that combines multiple conditions using logical operators (AND, OR, NOT).
For example, the following query retrieves all rows from the "Customers" table where the "City" column contains the value "London":
WHERE City = 'London';
You can also use the
WHERE operator to filter data based on a range of values. For example, the following query retrieves all rows from the "Orders" table where the "OrderDate" column is between two dates:
WHERE OrderDate BETWEEN '2022-01-01' AND '2022-12-31';
In addition to filtering data in a SELECT statement, the
WHERE operator can also be used in an
DELETE statement to modify or delete only the rows that meet a specified condition.
It's important to note that the
WHERE operator can have a significant impact on query performance, especially when working with large tables. To improve performance, you can use indexes on the columns being filtered, or use more specific conditions to narrow down the number of rows being returned.