Less06Using Subqueries to Solve Queries

After completing this lesson, you should be able to do the following:

Define subqueries

Describe the types of problems that subqueries can solve

After completing this lesson, you should be able to do the following:

Define subqueries

Describe the types of problems that subqueries can solve

The subquery (inner query) executes once before the main query (outer query).

The result of the subquery is used by the main query.

SELECT last_name

FROM   employees

WHERE  salary >

               (SELECT salary

                FROM   employees

                WHERE  last_name = 'Abel');

Enclose subqueries in parentheses.

Place subqueries on the right side of the comparison condition.

The ORDER BY clause in the subquery is not needed unless you are performing Top-N analysis.

Use single-row operators with single-row subqueries, and use multiple-row operators with
multiple-row subqueries.

SELECT last_name, job_id, salary

FROM   employees

WHERE  job_id = 

                (SELECT job_id

                 FROM   employees

                 WHERE  employee_id = 141)

AND    salary >

                (SELECT salary

                 FROM   employees

                 WHERE  employee_id = 143);

SELECT last_name, job_id, salary

FROM   employees

WHERE  salary =

                (SELECT MIN(salary)

                 FROM   employees);

•The Oracle server executes subqueries first.

•The Oracle server returns results into the HAVING clause of the main query.

SELECT   department_id, MIN(salary)

FROM     employees

GROUP BY department_id

HAVING   MIN(salary) >

                       (SELECT MIN(salary)

                        FROM   employees

                        WHERE  department_id = 50);

SELECT employee_id, last_name

FROM   employees

WHERE  salary =

                (SELECT   MIN(salary)

                 FROM     employees

                 GROUP BY department_id);

Return more than one row

Use multiple-row comparison operators

SELECT employee_id, last_name, job_id, salary

FROM   employees

WHERE  salary < ANY

                    (SELECT salary

                     FROM   employees

                     WHERE  job_id = 'IT_PROG')

AND    job_id <> 'IT_PROG';

SELECT employee_id, last_name, job_id, salary

FROM   employees

WHERE  salary < ALL

                    (SELECT salary

                     FROM   employees

                     WHERE  job_id = 'IT_PROG')

AND    job_id <> 'IT_PROG';

SELECT emp.last_name

FROM   employees emp

WHERE  emp.employee_id NOT IN

                           (SELECT mgr.manager_id

                            FROM   employees mgr);

This practice covers the following topics:

Creating subqueries to query values based on unknown criteria

Using subqueries to find out which values exist in one set of data and not in another

Less05Displaying Data

After completing this lesson, you should be able to do the following:

Write SELECT statements to access data from more than one table using equijoins and non-equijoins

Join a table to itself by using a self-join

View data that generally does not meet a join condition by using outer joins

Generate a Cartesian product of all rows from two or more tables

Joins that are compliant with the SQL:1999 standard include the following:

Cross joins

Natural joins

USING clause

Full (or two-sided) outer joins

Arbitrary join conditions for outer joins

The NATURAL JOIN clause is based on all columns in the two tables that have the same name.

It selects rows from the two tables that have equal values in all matched columns.

If the columns having the same names have different data types, an error is returned.

SELECT department_id, department_name,

       location_id, city

FROM   departments

NATURAL JOIN locations ;

If several columns have the same names but the data types do not match, the NATURAL JOIN clause can be modified with the USING clause to specify the columns that should be used for an equijoin.

Use the USING clause to match only one column when more than one column matches.

Do not use a table name or alias in the referenced columns.

SELECT employees.employee_id, employees.last_name,

       departments.location_id, department_id

FROM   employees JOIN departments

USING (department_id) ;

Use table prefixes to qualify column names that are in multiple tables.

Use table prefixes to improve performance.

Use column aliases to distinguish columns that have identical names but reside in different tables.

Do not use aliases on columns that are identified in the USING clause and listed elsewhere in the SQL statement.

Use table aliases to simplify queries.

Use table aliases to improve performance

SELECT e.employee_id, e.last_name,

       d.location_id, department_id

FROM   employees e JOIN departments d

USING (department_id) ;

The join condition for the natural join is basically an equijoin of all columns with the same name.

Use the ON clause to specify arbitrary conditions or specify columns to join.

The join condition is separated from other search conditions.

The ON clause makes code easy to understand.

SELECT e.employee_id, e.last_name, e.department_id,

       d.department_id, d.location_id

FROM   employees e JOIN departments d

ON     (e.department_id = d.department_id);

SELECT e.last_name emp, m.last_name mgr

FROM   employees e JOIN employees m

ON    (e.manager_id = m.employee_id);

SELECT e.employee_id, e.last_name, e.department_id,

       d.department_id, d.location_id

FROM   employees e JOIN departments d

ON     (e.department_id = d.department_id)

AND    e.manager_id = 149 ;

SELECT employee_id, city, department_name

FROM   employees e

JOIN   departments d

ON     d.department_id = e.department_id

JOIN   locations l

ON     d.location_id = l.location_id;

In SQL:1999, the join of two tables returning only matched rows is called an inner join.

A join between two tables that returns the results of the inner join as well as the unmatched rows from the left (or right) tables is called a left (or right) outer join.

A join between two tables that returns the results of an inner join as well as the results of a left and right join is a full outer join.

SELECT e.last_name, e.department_id, d.department_name

FROM   employees e LEFT OUTER JOIN departments d

ON   (e.department_id = d.department_id) ;

SELECT e.last_name, e.department_id, d.department_name

FROM   employees e RIGHT OUTER JOIN departments d

ON    (e.department_id = d.department_id) ;

SELECT e.last_name, d.department_id, d.department_name

FROM   employees e FULL OUTER JOIN departments d

ON   (e.department_id = d.department_id) ;

A Cartesian product is formed when:

A join condition is omitted

A join condition is invalid

All rows in the first table are joined to all rows in the second table

To avoid a Cartesian product, always include a valid join condition.

The CROSS JOIN clause produces the cross-product of two tables.

This is also called a Cartesian product between the two tables. 

SELECT last_name, department_name

FROM   employees

CROSS JOIN departments ;

In this lesson, you should have learned how to use joins to display data from multiple tables by using:

Equijoins

Non-equijoins

Outer joins

Self-joins

Cross joins

Natural joins

Full (or two-sided) outer joins

This practice covers the following topics:

Joining tables using an equijoin

Performing outer and self-joins

Adding conditions