Interactive SQL Tutorial

3. Conditional Selection

To further discuss the SELECT statement, let's look at a new example table (for hypothetical purposes only):

EmployeeStatisticsTable
EmployeeIDNo	Salary	Benefits	Position
010	75000	15000	Manager
105	65000	15000	Manager
152	60000	15000	Manager
215	60000	12500	Manager
244	50000	12000	Staff
300	45000	10000	Staff
335	40000	10000	Staff
400	32000	7500	Entry-Level
441	28000	7500	Entry-Level

 Relational Operators

There are six Relational Operators in SQL, and after introducing them, we'll see how they're used:

=	Equal
<> or != (see manual)	Not Equal
<	Less Than
>	Greater Than
<=	Less Than or Equal To
>=	Greater Than or Equal To

The WHERE clause is used to specify that only certain rows of the table are displayed, based on the criteria described in that WHERE clause. It is most easily understood by looking at a couple of examples.

If you wanted to see the EMPLOYEEIDNO's of those making at or over $50,000, use the following:

SELECT EMPLOYEEIDNO

FROM EMPLOYEESTATISTICSTABLE

WHERE SALARY >= 50000

Notice that the >= (greater than or equal to) sign is used, as we wanted to see those who made greater than $50,000, or equal to $50,000, listed together. This displays:

EMPLOYEEIDNO
------------
010
105
152
215
244

The WHERE description, SALARY >= 50000, is known as a condition. The same can be done for text columns:

SELECT EMPLOYEEIDNO

FROM EMPLOYEESTATISTICSTABLE

WHERE POSITION = 'Manager'

This displays the ID Numbers of all Managers. Generally, with text columns, stick to equal to or not equal to, and make sure that any text that appears in the statement is surrounded by single quotes (').

More Complex Conditions: Compound Conditions

The AND operator joins two or more conditions, and displays a row only if that row's data satisfies ALL conditions listed (i.e. all conditions hold true). For example, to display all staff making over $40,000, use:

SELECT EMPLOYEEIDNO

FROM EMPLOYEESTATISTICSTABLE

WHERE SALARY > 40000 AND POSITION = 'Staff'

The OR operator joins two or more conditions, but returns a row if ANY of the conditions listed hold true. To see all those who make less than $40,000 or have less than $10,000 in benefits, listed together, use the following query:

SELECT EMPLOYEEIDNO

FROM EMPLOYEESTATISTICSTABLE

WHERE SALARY < 40000 OR BENEFITS < 10000

AND & OR can be combined, for example:

SELECT EMPLOYEEIDNO

FROM EMPLOYEESTATISTICSTABLE

WHERE POSITION = 'Manager' AND SALARY > 60000 OR BENEFITS > 12000

First, SQL finds the rows where the salary is greater than $60,000 and the position column is equal to Manager, then taking this new list of rows, SQL then sees if any of these rows satisfies the previous AND condition or the condition that the Benefits column is greater then $12,000. Subsequently, SQL only displays this second new list of rows, keeping in mind that anyone with Benefits over $12,000 will be included as the OR operator includes a row if either resulting condition is True. Also note that the AND operation is done first.

To generalize this process, SQL performs the AND operation(s) to determine the rows where the AND operation(s) hold true (remember: all of the conditions are true), then these results are used to compare with the OR conditions, and only display those remaining rows where the conditions joined by the OR operator hold true.

To perform OR's before AND's, like if you wanted to see a list of employees making a large salary (>$50,000) or have a large benefit package (>$10,000), and that happen to be a manager, use parentheses:

SELECT EMPLOYEEIDNO

FROM EMPLOYEESTATISTICSTABLE

WHERE POSITION = 'Manager' AND (SALARY > 50000 OR BENEFITS > 10000)

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