tailieunhanh - Advanced Subqueries

After completing this lesson, you should be able to do the following: Write a multiple-column subquery Describe and explain the behavior of subqueries when null values are retrieved Write a subquery in a FROM clause Use scalar subqueries in SQL Describe the types of problems that can be solved with correlated subqueries Write correlated subqueries Update and delete rows using correlated subqueries Use the EXISTS and NOT EXISTS operators Use the WITH clause | Advanced Subqueries Schedule: Timing Topic 60 minutes Lecture 50 minutes Practice 110 minutes Total Objectives After completing this lesson, you should be able to do the following: Write a multiple-column subquery Describe and explain the behavior of subqueries when null values are retrieved Write a subquery in a FROM clause Use scalar subqueries in SQL Describe the types of problems that can be solved with correlated subqueries Write correlated subqueries Update and delete rows using correlated subqueries Use the EXISTS and NOT EXISTS operators Use the WITH clause Lesson Aim In this lesson, you learn how to write multiple-column subqueries and subqueries in the FROM clause of a SELECT statement. You also learn how to solve problems by using scalar, correlated subqueries and the WITH clause. What Is a Subquery? A subquery is a SELECT statement embedded in a clause of another SQL statement. SELECT . FROM . WHERE . (SELECT . FROM . WHERE .) Main query Subquery What Is a . | Advanced Subqueries Schedule: Timing Topic 60 minutes Lecture 50 minutes Practice 110 minutes Total Objectives After completing this lesson, you should be able to do the following: Write a multiple-column subquery Describe and explain the behavior of subqueries when null values are retrieved Write a subquery in a FROM clause Use scalar subqueries in SQL Describe the types of problems that can be solved with correlated subqueries Write correlated subqueries Update and delete rows using correlated subqueries Use the EXISTS and NOT EXISTS operators Use the WITH clause Lesson Aim In this lesson, you learn how to write multiple-column subqueries and subqueries in the FROM clause of a SELECT statement. You also learn how to solve problems by using scalar, correlated subqueries and the WITH clause. What Is a Subquery? A subquery is a SELECT statement embedded in a clause of another SQL statement. SELECT . FROM . WHERE . (SELECT . FROM . WHERE .) Main query Subquery What Is a Subquery? A subquery is a SELECT statement that is embedded in a clause of another SQL statement, called the parent statement. The subquery (inner query) returns a value that is used by the parent statement. Using a nested subquery is equivalent to performing two sequential queries and using the result of the inner query as the search value in the outer query (main query). Subqueries can be used for the following purposes: To provide values for conditions in WHERE, HAVING, and START WITH clauses of SELECT statements To define the set of rows to be inserted into the target table of an INSERT or CREATE TABLE statement To define the set of rows to be included in a view or snapshot in a CREATE VIEW or CREATE SNAPSHOT statement To define one or more values to be assigned to existing rows in an UPDATE statement To define a table to be operated on by a containing query. (You do this by placing the subquery in the FROM clause. This can be done in INSERT, UPDATE, and DELETE statements as well.) .