RELATIONAL DATA RETRIEVAL: SQL - Fundamentals of Database Management Systems - page 76

SELECT CUSTNUM, CUSTNAME, HQCITY
FROM CUSTOMER
WHERE HQCITY='New York'
OR CUSTNUM > 1500;

results in:

CUSTNUM CUSTNAME HQCITY

0121 Main St. Hardware New York

1525 Fred's Tool Stores Atlanta

1700 XYZ Stores Washington

1826 City Hardware New York

2198 Western Hardware New York

2267 Central Stores New York

Notice that the OR operator really means one or the other or both .Customer
0121 is included because it is headquartered in New York. Customers 1525 and 1700
are included because they have customer numbers higher than 1500. Customers
1826, 2198, and 2267 are included because they satisfy both conditions.

Both AND and OR What if both AND and OR are specified in the same WHERE
clause? AND is said to be ''higher in precedence'' than OR, and so all ANDs are
considered before any ORs are considered. The following query, which has to be
worded very carefully, illustrates this point:

''List the customer numbers, customer names, and headquarters cities of the
customers that are headquartered in New York or that satisfy the two conditions
of having a customer number higher than 1500 and being headquartered in
Atlanta.''

SELECT CUSTNUM, CUSTNAME, HQCITY
FROM CUSTOMER
WHERE HQCITY='New York'
OR CUSTNUM > 1500
AND HQCITY='Atlanta';

The result of this query is:

CUSTNUM CUSTNAME HQCITY

0121 Main St. Hardware New York

1525 Fred's Tool Stores Atlanta

1826 City Hardware New York

2198 Western Hardware New York

2267 Central Stores New York

Notice that since the AND is considered first , one way for a row to qualify
for the result is if its customer number is greater than 1500 and its headquarters city
is Atlanta. With the AND taken first, it's that combination or the headquarters city
has to be New York. Considering the OR operator first would change the whole