The <sql:transaction> tag in JSTL (JavaServer Pages Standard Tag Library) is used to group multiple SQL operations into a single transaction. 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"/>

<sql:transaction dataSource="${myDataSource}">
  <sql:update>
    INSERT INTO mytable (column1, column2) VALUES ('value1', 'value2')
  </sql:update>
  <sql:update>
    UPDATE mytable SET column1 = 'newvalue' WHERE column2 = 'value2'
  </sql:update>
</sql:transaction>

In this example, we first define a data source using the <sql:setDataSource> tag. We then use the <sql:transaction> tag to group two SQL operations, an INSERT and an UPDATE, into a single transaction. If either operation fails, the entire transaction will be rolled back, ensuring that the database is left in a consistent state.

Note that the <sql:transaction> tag is not supported in all database systems, and its behavior may depend on the database system and JDBC driver being used.

Also 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.