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Using Comparison Operators in a WHERE Clause
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You can use a number of different comparison operators in a WHERE clause (see Table 3-1).
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Table 3-1. Comparison Operators Operator
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Equals Less than Greater than
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EmployeeID = 1 EmployeeID < 1 EmployeeID > 1
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CHAPTER 3 s INTRODUCING SQL
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Table 3-1. Continued Operator
<= >= <>,!= !< !>
Description
Less than or equal to Greater than or equal to Not equal to Not less than Not greater than
Example
EmployeeID <= 1 EmployeeID >= 1= EmployeeID <> 1 EmployeeID !< 1 EmployeeID !> 1
s As mentioned earlier, every database vendor has its own implementation of SQL. This discussion Tip
is specific to T-SQL; for example, standard SQL doesn t have the != operator, and calls <> the not equals operator. In fact, standard SQL calls the expressions in a WHERE clause predicates; we ll use that term because predicates are either true or false, but other expressions don t have to be. If you work with another version of SQL, please refer to its documentation for specifics.
In addition to these operators, the LIKE operator allows you to match patterns in character data (see Table 3-2). As with all SQL character data, strings must be enclosed in single quotes (').
Table 3-2. The LIKE Operator Operator
LIKE
Description
Allows you to specify a pattern to search for
Example
WHERE Title LIKE 'Sales %' selects all rows where the Title column contains a value that starts with the word Sales and a space.
You can use four different wildcards in the pattern (see Table 3-3).
Table 3-3. Wildcard Characters Wildcard
% _ [ ] [^]
Description
Any combination of characters. Any one character. WHERE Title LIKE '_ales' selects all rows where the Title column equals Aales, aales, Bales, bales, and so on. A single character within a range [a-d] or set [abcd]. WHERE Title LIKE '[bs]ales' selects all rows where the Title column equals either the word bales or sales. A single character not within a range [^a-d] or set [^abcd].
CHAPTER 3 s INTRODUCING SQL
Sometimes it s useful to select rows where a value is unknown. When no value has been assigned to a column, the column is NULL. (This isn t the same as a column that contains the value 0 or a blank.) To select a row with a column that s NULL, use the IS [NOT] NULL operator (see Table 3-4).
Table 3-4. The IS [NOT] NULL Operator Operator
IS NULL IS NOT NULL
Description
Allows you to select rows where a column has no value. Allows you to select rows where a column has a value.
Example
WHERE Region IS NULL returns all rows where Region has no value. WHERE Region IS NOT NULL returns all rows where Region has a value.
s Note You must use the IS
NULL and IS NOT NULL operators (collectively called the null predicate in standard SQL) to select or exclude NULL column values, respectively. The following is a valid query but always produces zero rows: SELECT * FROM employees WHERE region = NULL. If you change = to IS, the query will return rows where regions have no value.
To select values in a range or in a set, you can use the BETWEEN and IN operators (see Table 3-5).
Table 3-5. The BETWEEN and IN Operators Operator
BETWEEN
Description
True if a value is within a range.
Example
WHERE extension BETWEEN 400 AND 500 returns the rows where Extension is between 400 and 500, inclusive. WHERE city IN ('Seattle', 'London') returns the rows where City is either Seattle or London.
True if a value is in a list. The list can be the result of a subquery.
Combining Predicates
You ll often need to use more than one predicate to filter your data. You can use the logical operators shown in Table 3-6.
CHAPTER 3 s INTRODUCING SQL
Table 3-6. SQL Logical Operators Operator
Description
Combines two expressions, evaluating the complete expression as true only if both are true Negates a Boolean value Combines two expressions, evaluating the complete expression as true if either is true
Example
WHERE (title LIKE 'Sales%' AND lastname ='Peacock') WHERE NOT (title LIKE 'Sales%' AND lastname = 'Peacock') WHERE (title = 'Peacock' OR title = 'King')
NOT OR
When you use these operators, it s often a good idea to use parentheses to clarify the conditions. In complex queries, this might be absolutely necessary.
Sorting Data
After you ve filtered the data you want, you can sort the data by one or more columns and in a certain direction. Because tables are by definition unsorted, the order in which rows are retrieved by a query is unpredictable. To impose an ordering, you use the ORDER BY clause:
ORDER BY <column> [ASC | DESC] {, n}
The <column> is the column that should be used to sort the result. The {, n} syntax means you can specify any number of columns separated by commas. The result will be sorted in the order in which you specify the columns. The following are the two sort directions: ASC: Ascending (1, 2, 3, 4, and so on). DESC: Descending (10, 9, 8, 7, and so on). If you omit the ASC or DESC keywords, the sort order defaults to ASC. Now you ve seen the following basic syntax for queries:
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