The <sql:setDataSource> tag in JSTL (JavaServer Pages Standard Tag Library) is used to define a database connection and make it available to other SQL tags. Here's an example of how to use it:

<%@ taglib uri="http://java.sun.com/jsp/jstl/sql" prefix="sql" %>

<sql:setDataSource
  var="myDataSource"
  driver="com.mysql.jdbc.Driver"
  url="jdbc:mysql://localhost:3306/mydatabase"
  user="myuser"
  password="mypassword"/>

In this example, we define a data source named myDataSource using the <sql:setDataSource> tag. The driver attribute specifies the JDBC driver class name for the database, and the url attribute specifies the database connection URL. The user and password attributes specify the username and password to use for the database connection.

Once the data source is defined, we can use it in other SQL tags, such as <sql:query> or <sql:update>. For example:

<sql:query dataSource="${myDataSource}" var="myQueryResult">
  SELECT * FROM mytable;
</sql:query>

In this example, we use the dataSource attribute to specify the data source to use for the query. The var attribute specifies the name of a variable to store the query result in.

Note that JSTL SQL tags have been deprecated since JSTL 1.2, so it is recommended to use a different method for database access, such as JDBC or an ORM (Object-Relational Mapping) framework like Hibernate.