WHERE Clauses in SQL
WHERE clause in a
statement provides the criteria for selecting values. For example, in the
following code fragment, values will be selected only if they occur in a row in
which the column Last_Name begins with the string 'Washington'.
SELECT First_Name, Last_Name FROM Employees WHERE Last_Name LIKE 'Washington%'
LIKE is used to compare strings, and it
offers the feature that patterns containing wildcards can be used. For example,
in the code fragment above, there is a percent sign (%) at the end of
'Washington', which signifies that any value containing the string 'Washington'
plus zero or more additional characters will satisfy this selection criterion.
So 'Washington' or 'Washingtonian' would be matches, but 'Washing' would not be.
The other wildcard used in
LIKE clauses is an underbar
(_), which stands for any one character. For example,
WHERE Last_Name LIKE 'Ba_man'
would match 'Batman', 'Barman', 'Badman', 'Balman', 'Bagman', 'Bamman', and so on.
The code fragment below has a
WHERE clause that uses
the equal sign (=) to compare numbers. It selects the first and last name of the
employee who is assigned car 12.
SELECT First_Name, Last_Name FROM Employees WHERE Car_Number = 12
The next code fragment selects the first and last names of employees whose employee number is greater than 10005:
SELECT First_Name, Last_Name FROM Employees WHERE Employee_Number > 10005
WHERE clauses can get rather elaborate, with
multiple conditions and, in some DBMSs, nested conditions. This overview will
not cover complicated
WHERE clauses, but the following
code fragment has a
WHERE clause with two conditions;
this query selects the first and last names of employees whose employee number
is less than 10100 and who do not have a company car.
SELECT First_Name, Last_Name FROM Employees WHERE Employee_Number < 10100 and Car_Number IS NULL
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