The COLLATE clause in SQL is used to specify or override the collation of a column or expression. Collation determines how string comparison is performed, influencing aspects like case sensitivity and accent sensitivity for string data. You can use the COLLATE clause in several contexts, primarily in the SELECT, ORDER BY, or WHERE clauses.
Here's how to use the COLLATE clause to change the collation of a column or expression:
Basic Syntax
SELECT column_name COLLATE collation_name
FROM table_name
WHERE column_name COLLATE collation_name = 'value'
ORDER BY column_name COLLATE collation_name;
Example
Suppose you have a table named Users with a column Name which uses a default collation that is case-sensitive. If you want to perform a case-insensitive search or display in a specific collation, you can do the following:
SELECT Name COLLATE SQL_Latin1_General_CP1_CI_AS AS Name_CaseInsensitive
FROM Users
WHERE Name COLLATE SQL_Latin1_General_CP1_CI_AS = 'john doe'
ORDER BY Name COLLATE SQL_Latin1_General_CP1_CI_AS;
Key Points
1. Collations: Make sure you know the available collations in your database system. Common collations include:
- SQL_Latin1_General_CP1_CI_AS (Case Insensitive, Accent Sensitive)
- SQL_Latin1_General_CP1_CS_AS (Case Sensitive, Accent Sensitive)
2. Data Type Compatibility: When using COLLATE, the specified collation should be compatible with the data type of the column or expression.
3. Performance Considerations: Keep in mind that using COLLATE can affect query performance, especially if it changes the collation on indexed columns.
4. Usage in Joins: You might also need to specify collation in join conditions if collations conflict. For instance:
SELECT *
FROM Users u
JOIN Orders o ON u.Name COLLATE SQL_Latin1_General_CP1_CI_AS = o.CustomerName COLLATE SQL_Latin1_General_CP1_CI_AS;
By using the COLLATE clause appropriately, you can ensure that your queries behave as expected with respect to string comparisons based on collation rules.