Examples of subselect queries

The following examples illustrate the susbelect query.

Example 1: Select all columns and rows from the EMPLOYEE table.
```
  SELECT * FROM EMPLOYEE
```
Example 2: 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 3: 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 1955.
```
  SELECT EMPNO, LASTNAME, WORKDEPT, DEPTNAME 
     FROM EMPLOYEE, DEPARTMENT                     
     WHERE WORKDEPT = DEPTNO     
     AND YEAR(BIRTHDATE) < 1955
```
Example 4: 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 5: 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 6: 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 is executed once in this example.

Example 7: 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 6, the subquery in the HAVING clause is executed for each group.

Example 8: 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) to get the AVGSALARY and EMPCOUNT columns, and 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 processing resources 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 are considered by the view.
Example 9: 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
```

Example 10: Query the EMP_ACT table and return those project numbers that have an employee whose salary is in the top 10 of all employees.

  SELECT EMP_ACT.EMPNO,PROJNO
     FROM EMP_ACT
     WHERE EMP_ACT.EMPNO IN
         (SELECT EMPLOYEE.EMPNO
          FROM EMPLOYEE
          ORDER BY SALARY DESC
          FETCH FIRST 10 ROWS ONLY)

Example 11: Assuming that PHONES and IDS are two SQL variables with array values of the same cardinality, turn these arrays into a table with three columns (one for each array and one for the position), and one row per array element.
```
   SELECT T.PHONE, T.ID, T.INDEX FROM UNNEST(PHONES, IDS)
     WITH ORDINALITY AS T(PHONE, ID, INDEX)
     ORDER BY T.INDEX
```