In relational algebra, the projection operation will retrieve a specified subset of columns from a given table. It will retrieve all of the rows but only the specified columns. The following statement illustrates the SQL format for an implementation of projection:

Using the table named 'EMPLOYEE' and the columns 'NAME' and 'EMPNO', retrieve all the data. The SQL SELECT command would be as follows:

Note: Case is not important when entering column names, but spelling needs to match exactly the name of the column in the table.

The results table should look like this:

Since order was not specified, the rows were returned in the order in which they reside within the table. It just so happens that they are in EMPNO order.

The SQL SELECT command is used to implement both the selection and projection operations. Most of the time your queries will combine selection and projection to retrieve data. The example below combines the selection and projection operations from the previous illustrations:

The results table is as follows:

For you to try:

1. Write a SQL query that will return the employee number and name of all clerks, ordered by employee number.
2. Write a SQL query that will return the name and job of all employees who are male, ordered alphabetically by name.
3. Type the command to return a description of the DEPT table. Return to SQL Exercise 1 if you don't remember the command syntax. Write a SQL query that will return the department name and location of all departments, ordered alphabetically by department name.

Go here to check your answers.

Last revised on 10/21/97 by J. Warren, Kennesaw State University

Department of Computer Science and Information Systems

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