SELECT Statements

Lecture Notes

Sree Nilakanta

Fall 2010 (rev)

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SELECT Statements

• Retrieve data from one or more tables, object tables, views, object views, or snapshots

• Must have SELECT privilege

• Examples

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Examples

• Selects rows from the employee table with the department number of 40.

• SELECT * FROM emp WHERE deptno = 40

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Examples

• selects the name, job, salary and department number of all employees except salesmen from department number 30:

• SELECT ename, job, sal, deptno FROM emp WHERE NOT (job = 'SALESMAN' AND deptno = 30)

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Examples

• Selects from subqueries in the FROM clause and gives departments total employees and salaries as a percentage of all the departments:

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SELECT a.deptno "Department", a.num_emp/b.total_count "%Employees", a.sal_sum/b.total_sal "%Salary”FROM(SELECT deptno, COUNT(*) num_emp, SUM(SAL) sal_sumFROM scott.empGROUP BY deptno) a,(SELECT COUNT(*) total_count, SUM(sal) total_salFROM scott.emp) b ;

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Group by Examples

• Return the minimum and maximum salaries for each department in the employee table, issue the following statement.

• SELECT deptno, MIN(sal), MAX(sal)FROM emp GROUP BY deptno;

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Group by examples

• Return the minimum and maximum salaries for the clerks in each department, issue the following statement.

• SELECT deptno, MIN(sal), MAX(sal) FROM emp WHERE job = 'CLERK’GROUP BY deptno;

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HAVING Clause

• Restrict which groups of rows defined by the GROUP BY clause. Oracle

• removes all rows that do not satisfy WHERE clause,

• calculates and forms the groups as specified in the GROUP BY clause,

• removes all groups that do not satisfy the HAVING clause.

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Having clause

• Return the minimum and maximum salaries for the clerks in each department whose lowest salary is below $1,000.

• SELECT deptno, MIN(sal), MAX(sal) FROM emp WHERE job = 'CLERK' GROUP BY deptno HAVING MIN(sal) < 1000;

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ORDER BY Clause

• order the rows selected by a query

• can specify multiple expressions in the ORDER BY clause

• sorts rows based on their values for the first expression

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Order by clause restrictions

• compound queries (containing set operators UNION, INTERSECT, MINUS, or UNION ALL), the ORDER BY clause must use positions, rather than explicit expressions

• cannot appear in subqueries within other statements

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Order by examples

• Select all salesmen's records from EMP, and order the results by commission in descending order.

• SELECT * FROM emp WHERE job = 'SALESMAN' ORDER BY comm DESC;

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Order by examples

• Select the employees from EMP ordered first by ascending department number and then by descending salary.

• SELECT ename, deptno, sal FROM emp ORDER BY deptno ASC, sal DESC;

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Order by examples

• Use the positional ORDER BY notation

• SELECT ename, deptno, sal FROM emp ORDER BY 2 ASC, 3 DESC;

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Joins

• A query that combines rows from two or more tables, views, or snapshots

• Performs a join whenever multiple tables appear in the query's FROM clause

• List any columns from any of these tables

• If any these tables have a common column name, these columns must be prefixed with table names to avoid ambiguity.

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Joins Examples

• Returns the name and job of each employee and the number and name of the department in which the employee works.

• SELECT ename, job, dept.deptno, dname FROM emp, dept WHERE emp.deptno = dept.deptno;

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Joins examples

• Returns the name, job, department number, and department name of all clerks.

• SELECT ename, job, dept.deptno, dname FROM emp, dept WHERE emp.deptno = dept.deptno AND job = 'CLERK';

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Joins: Self Joins

• Returns the name of each employee along with the name of the employee's manager.

• SELECT e1.ename||' works for '||e2.ename "Employees and their Managers" FROM emp e1, emp e2 WHERE e1.mgr = e2.empno;

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Joins: Cartesian Products

• have no join condition

• combines each row of one table with each row of the other

• generates many rows and is rarely useful