【SQL】SQL Reference Examples of subselects

SQL Reference
Examples of subselects


Example A1: Select all columns and rows from the EMPLOYEE table.

SELECT * FROM EMPLOYEE

Example A2: Join the EMP_ACT and EMPLOYEE tables, select all the columns from the EMP_ACT table and add the employee's surname (LASTNAME) from the EMPLOYEE table to each row of the result.

SELECT EMP_ACT.*, LASTNAME
FROM EMP_ACT, EMPLOYEE
WHERE EMP_ACT.EMPNO = EMPLOYEE.EMPNO

Example A3: Join the EMPLOYEE and DEPARTMENT tables, select the employee number (EMPNO), employee surname (LASTNAME), department number (WORKDEPT in the EMPLOYEE table and DEPTNO in the DEPARTMENT table) and department name (DEPTNAME) of all employees who were born (BIRTHDATE) earlier than 1930.

SELECT EMPNO, LASTNAME, WORKDEPT, DEPTNAME
FROM EMPLOYEE, DEPARTMENT
WHERE WORKDEPT = DEPTNO
AND YEAR(BIRTHDATE) < 1930

Example A4: Select the job (JOB) and the minimum and maximum salaries (SALARY) for each group of rows with the same job code in the EMPLOYEE table, but only for groups with more than one row and with a maximum salary greater than or equal to 27000.

SELECT JOB, MIN(SALARY), MAX(SALARY)
FROM EMPLOYEE
GROUP BY JOB
HAVING COUNT(*) > 1
AND MAX(SALARY) >= 27000

Example A5: Select all the rows of EMP_ACT table for employees (EMPNO) in department (WORKDEPT) 'E11'. (Employee department numbers are shown in the EMPLOYEE table.)

SELECT *
FROM EMP_ACT
WHERE EMPNO IN
(SELECT EMPNO
FROM EMPLOYEE
WHERE WORKDEPT = 'E11')

Example A6: From the EMPLOYEE table, select the department number (WORKDEPT) and maximum departmental salary (SALARY) for all departments whose maximum salary is less than the average salary for all employees.

SELECT WORKDEPT, MAX(SALARY)
FROM EMPLOYEE
GROUP BY WORKDEPT
HAVING MAX(SALARY) < (SELECT AVG(SALARY)
FROM EMPLOYEE)

The subquery in the HAVING clause would only be executed once in this example.

Example A7: Using the EMPLOYEE table, select the department number (WORKDEPT) and maximum departmental salary (SALARY) for all departments whose maximum salary is less than the average salary in all other departments.

SELECT WORKDEPT, MAX(SALARY)
FROM EMPLOYEE EMP_COR
GROUP BY WORKDEPT
HAVING MAX(SALARY) < (SELECT AVG(SALARY)
FROM EMPLOYEE
WHERE NOT WORKDEPT = EMP_COR.WORKDEPT)

In contrast to Example A6, the subquery in the HAVING clause would need to be executed for each group.

Example A8: Determine the employee number and salary of sales representatives along with the average salary and head count of their departments.

This query must first create a nested table expression (DINFO) in order to get the AVGSALARY and EMPCOUNT columns, as well as the DEPTNO column that is used in the WHERE clause.

SELECT THIS_EMP.EMPNO, THIS_EMP.SALARY, DINFO.AVGSALARY, DINFO.EMPCOUNT
FROM EMPLOYEE THIS_EMP,
(SELECT OTHERS.WORKDEPT AS DEPTNO,
AVG(OTHERS.SALARY) AS AVGSALARY,
COUNT(*) AS EMPCOUNT
FROM EMPLOYEE OTHERS
GROUP BY OTHERS.WORKDEPT
) AS DINFO
WHERE THIS_EMP.JOB = 'SALESREP'
AND THIS_EMP.WORKDEPT = DINFO.DEPTNO

Using a nested table expression for this case saves the overhead of creating the DINFO view as a regular view. During statement preparation, accessing the catalog for the view is avoided and, because of the context of the rest of the query, only the rows for the department of the sales representatives need to be considered by the view.

Example A9: Display the average education level and salary for 5 random groups of employees.

This query requires the use of a nested table expression to set a random value for each employee so that it can subsequently be used in the GROUP BY clause.

SELECT RANDID , AVG(EDLEVEL), AVG(SALARY)
FROM ( SELECT EDLEVEL, SALARY, INTEGER(RAND()*5) AS RANDID
FROM EMPLOYEE
) AS EMPRAND
GROUP BY RANDID

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