Oracle Series Seven inquiry

Sub-query syntax

SELECT    select_list
FROM    table
WHERE    expr operator
             (SELECT    select_list
                FROM        table);

• The subquery (inner query) is performed once before the main query.
• Results of the subquery by the main query (outer query) used.

Example:

SELECT
    last_name
FROM
    employees
WHERE
    salary > (
        SELECT
            salary
        FROM
            employees
        WHERE
            last_name = 'Abel'
    );

Precautions

• Subquery to be included in parentheses.

• The subquery on the right side of the comparison condition.

• Corresponding to one-line single-row subquery operators, operators corresponding to a plurality of rows multiple rows subqueries.

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Subqueries type



single-row subquery

• Returns only one row.
• Using single row comparison operators.

Example:

• Job_id returned with the same number of 141 employees, salary of employees and more than 143 employee name, job_id and wages

 

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
    );

The use of group functions in a subquery

• Returns the minimum wage of employees of the company last_name, job_id, and salary

SELECT
    last_name,
    job_id,
    salary
FROM
    employees
WHERE
    salary = (
        SELECT
            MIN(salary)
        FROM
            employees
    );

HAVING clause subquery

• First, the implementation of sub-queries.
• Return results to the main query in the HAVING clause.

 

• Topic: Query No. 50 is greater than the minimum wage sector minimum wage and the minimum wage department id

SELECT
    department_id,
    MIN(salary)
FROM
    employees
GROUP BY
    department_id
HAVING
    MIN(salary) > (
        SELECT
            MIN(salary)
        FROM
            employees
        WHERE
            department_id = 50
    );

Illegal use subqueries

Null sub-query problem

SELECT
    last_name,
    job_id
FROM
    employees
WHERE
    job_id = (
        SELECT
            job_id
        FROM
            employees
        WHERE
            last_name = 'Haas'
    );

Multiple-row subqueries

• Return multiple rows.
• Using multiple-row comparison operators.

ANY operator using the multi-row subqueries

Example: the return of any member of the other departments of a low-wage employees as 'IT_PROG' department than job_id

              Job number, name, job_id, and salary

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';

Use ALL operators in a multi-line subquery

example: Return to other sectors than job_id 'IT_PROG' department employees all wages are low
            employee number, name, salary and job_id

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';

Null sub-query problem

SELECT
    emp.last_name
FROM
    employees emp
WHERE
    emp.employee_id NOT IN (
        SELECT
            mgr.manager_id
        FROM
            employees mgr
    );