Tài liệu Using SQL phần 2 docx

For example, the following SELECT statement uses the IN operator to retrieve products with a ProductID of 1, 2, 5, 15, 20, 45, or 50: SELECT ProductID, ProductName, QuantityPerUnit, Unit

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Figure 3.6: Products where ProductName is like 'Cha%'

The next SELECT statement uses the LIKE operator to retrieve products where the ProductName column is like '[ABC]%':

SELECT ProductID, ProductName

FROM Products

WHERE ProductName LIKE '[ABC]%';

Figure 3.7 shows the results of this SELECT statement LIKE '[ABC]%' matches products with a name that starts with any of the letters A, B, or C, and ends with any number of characters

Figure 3.7: Products where ProductName is like '[ABC]%'

The next SELECT statement uses the LIKE operator to retrieve products where the ProductName column is like '[^ABC]%':

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FROM Products

WHERE ProductName LIKE '[^ABC]%';

Figure 3.8 shows the results of this SELECT statement LIKE '[^ABC]%' matches

products with names that don't start with any of the letters A, B, or C, and end with any number of characters

Figure 3.8: Products where ProductName is like '[^ABC]%'

The next SELECT statement uses the LIKE operator to retrieve products where the ProductName column is like '[A-E]%':

FROM Products

WHERE ProductName LIKE '[A-E]%';

Figure 3.9 shows the results of this SELECT statement LIKE '[A-E]%' matches products with names that start with any of the letters A through E, and end with any number of characters

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Figure 3.9: Products where ProductName is like '[A-E]%'

Specifying a List of Values

You use the IN operator in a WHERE clause to retrieve rows with columns that contain values in a specified list For example, the following SELECT statement uses the IN operator to retrieve products with a ProductID of 1, 2, 5, 15, 20, 45, or 50:

SELECT ProductID, ProductName, QuantityPerUnit, UnitPrice

FROM Products

WHERE ProductID IN (1, 2, 5, 15, 20, 45, 50);

Here's another example that displays the OrderID column from the Orders table for the

rows where the CustomerID column is in the list retrieved by a subquery; the subquery

retrieves the CustomerID column from the Customers table where the CompanyName is like 'Fu%':

SELECT OrderID

FROM Orders

WHERE CustomerID IN (

SELECT CustomerID

FROM Customers

WHERE CompanyName LIKE 'Fu%'

);

The results of the subquery are used in the outer query

Specifying a Range of Values

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You use the BETWEEN operator in a WHERE clause to retrieve rows with columns that contain values in a specified range For example, the following SELECT statement uses the BETWEEN operator to retrieve products with a ProductID between 1 and 12:

FROM Products

WHERE ProductID BETWEEN 1 AND 12;

Here's another example that displays the OrderID column for the rows from the Orders table where the OrderDate is between '1996-07-04' and '1996-07-08':

SELECT OrderID

FROM Orders

WHERE OrderDate BETWEEN '1996-07-04' AND '1996-07-08';

Reversing the Meaning of an Operator

You use the NOT keyword with an operator in a WHERE clause to reverse the meaning

of that operator For example, the following SELECT statement uses the NOT keyword

to reverse the meaning of the BETWEEN operator:

FROM Products

WHERE ProductID NOT BETWEEN 1 AND 12;

Note You can use the NOT keyword to reverse other operators, for example, NOT LIKE, NOT IN

Retrieving Rows with Columns Set to Null

Earlier, I mentioned that columns can contain null values A null value is different from a blank string or zero: A null value represents a value that hasn't been set, or is unknown You can use the IS NULL operator in a WHERE clause to determine if a column contains

a null value For example, the following SELECT statement uses the IS NULL operator

to retrieve customers where the Fax column contains a null value:

SELECT CustomerID, CompanyName, Fax

FROM Customers

WHERE Fax IS NULL;

Figure 3.10 shows the results of this SELECT statement

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Figure 3.10: Using the IS NULL operator to retrieve customers where Fax contains a null value

As you can see, null values are displayed as NULL in the Query Analyzer

Specifying Multiple Conditions

You can use the logical operators shown in Table 3.3 to specify multiple conditions in a WHERE clause

Table 3.3: LOGICAL OPERATORS

OPERATOR DESCRIPTION

a AND b Evaluates to true when a and b are both true

a OR b Evaluates to true when either a or b are true

NOT a Evaluates to true if a is false, and false if a is true

For example, the following SELECT statement uses the AND operator to retrieve

products where the UnitsInStock column is less than 10 and the ReorderLevel column is less than or equal to 20:

SELECT ProductID, ProductName, UnitsInStock, ReorderLevel

FROM Products

WHERE UnitsInStock < 10

AND ReorderLevel <= 20;

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Figure 3.11: Using the AND operator to retrieve products where UnitsInStock is less than

10 and ReorderLevel is less than or equal to 20

In the next example, the SELECT statement uses the OR operator to retrieve products where either the UnitsInStock column is less than 10 or the ReorderLevel column is less than or equal to 20:

FROM Products

WHERE UnitsInStock < 10

OR ReorderLevel <= 20;

Figure 3.12: Using the OR operator to retrieve products where either UnitsInStock is less than 10 or ReorderLevel is less than or equal to 20

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The next SELECT statement uses the NOT operator to retrieve products where the

UnitsInStock column is not less than 10:

FROM Products

WHERE NOT (UnitsInStock < 10);

Sorting Rows

You can use the ORDER BY clause to sort rows retrieved from the database You specify the column (or columns) to sort in the ORDER BY clause By default, rows are sorted in ascending order For example, the following SELECT statement orders the rows using the ProductName column:

FROM Products

ORDER BY ProductName;

Figure 3.13 shows the results of this SELECT statement As you can see, the rows are ordered in ascending order using the ProductName column

Figure 3.13: Using the ORDER BY clause to order products by ascending ProductName

You can explicitly state the order for a column using the ASC or DESC keyword ASC orders the columns in ascending order (smallest item first), and DESC orders the columns

in descending order (largest item first) For example, the following SELECT statement orders the products in descending order using the ProductName column:

FROM Products

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ORDER BY ProductName DESC;

You can specify multiple columns in an ORDER BY clause For example, the following SELECT statement orders the rows using both the UnitsInStock and ReorderLevel columns:

FROM Products

ORDER BY UnitsInStock DESC, ReorderLevel ASC;

Figure 3.14 shows the results of this SELECT statement As you can see, the rows are ordered by the UnitsInStock column first (in descending order), and then by the

ReorderLevel column (in ascending order)

Figure 3.14: Using the DESC and ASC keywords to order products by descending

UnitsInStock and ascending ReorderLevel

Retrieving the Top N Rows

You use the TOP keyword to just retrieve the top N rows from a SELECT statement For

example, the following SELECT statement uses the TOP keyword to retrieve the top 10 rows from the Products table, ordered by the ProductID column:

SELECT TOP 10 ProductID, ProductName, UnitsInStock, ReorderLevel

FROM Products

ORDER BY ProductID;

Note I've also used the optional ORDER BY clause in this example SELECT statement to order the rows by the ProductID column

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Figure 3.15: Using the TOP keyword to retrieve the top 10 products by ProductID

Eliminating Duplicate Rows

You use the DISTINCT keyword to eliminate duplicate rows retrieved by a SELECT statement For example, the following SELECT statement uses the DISTINCT keyword

to retrieve the distinct Country column values from the Customers table:

SELECT DISTINCT Country

FROM Customers;

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