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More Complex
Conditions: Compound Conditions / Logical Operators
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 than $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 any of the conditions joined by the OR operator hold
true (where a condition or result from an AND is paired with
another condition or AND result to use to evaluate the OR, which
evaluates to true if either value is true). Mathematically, SQL
evaluates all of the conditions, then evaluates the AND "pairs",
and then evaluates the OR's (where both operators evaluate left to
right).
To look at an example, for a given row for which the DBMS is
evaluating the SQL statement Where clause to determine whether to
include the row in the query result (the whole Where clause
evaluates to True), the DBMS has evaluated all of the conditions,
and is ready to do the logical comparisons on this result:
True AND False OR True AND True OR False AND False
First simplify the AND pairs:
False OR True OR False
Now do the OR's, left to right:
True OR False
True
The result is True, and the row passes the query conditions. Be
sure to see the next section on NOT's, and the order of logical
operations. Hopefully this section has helped you understand AND's
or OR's, as it's a difficult subject to explain briefly.
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);
IN & BETWEEN
An easier method of using compound conditions uses IN or
BETWEEN. For example, if you wanted to list all managers and
staff:
SELECT EMPLOYEEIDNO
FROM EMPLOYEESTATISTICSTABLE
WHERE POSITION IN ('Manager', 'Staff');
or to list those making greater than or equal to $30,000, but
less than or equal to $50,000, use:
SELECT EMPLOYEEIDNO
FROM EMPLOYEESTATISTICSTABLE
WHERE SALARY BETWEEN 30000 AND 50000;
To list everyone not in this range, try:
SELECT EMPLOYEEIDNO
FROM EMPLOYEESTATISTICSTABLE
WHERE SALARY NOT BETWEEN 30000 AND 50000;
Similarly, NOT IN lists all rows excluded from the IN
list.
Additionally, NOT's can be thrown in with AND's & OR's,
except that NOT is a unary operator (evaluates one condition,
reversing its value, whereas, AND's & OR's evaluate two
conditions), and that all NOT's are performed before any AND's or
OR's.
SQL Order of Logical Operations (each operates from left
to right)
NOT
AND
OR
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