The Language of SQL- P13 ppsx

Sorting DataKeywords Introduced: ORDER BY , ASC , DESC The ability to present data in a sorted order is often essential to the task at hand.. For example, if you were shown a large list

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Sorting Data

Keywords Introduced: ORDER BY ,

ASC , DESC

The ability to present data in a sorted order is often essential to the task at hand For example, if you were shown a large list of customers in a random order, you’d find it difficult to locate any one particular customer However, if the same list were sorted alphabetically, then you could quickly locate the desired customer The idea of sorting data alphabetically applies to many situations, even when the data isn’t strictly alphabetic in nature For example, you may want to sort a list of orders by the order date and time to allow you to rapidly find an order taken at a particular date and time Or you might want to sort a list of orders by the order amount, to allow you to view orders from the smallest to the largest No matter what particular form your sort takes, it adds a useful way to organize your data as

it is being presented to the end user

Adding a Sort

Up until now, data has not been returned in any particular order When a SELECT is issued, you never know which row will come first If the query is executed from within a software program, and no one ever sees the data at that point in time, then it really doesn’t matter But if the data is to be immediately displayed to a user, then the order of rows is often significant A sort can be added easily to aSELECTstatement by using anORDER BYclause

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Here’s the general format for aSELECTstatement with anORDER BYclause:

SELECT columnlist

FROM tablelist

ORDER BY columnlist

TheORDER BYclause is always after theFROMclause, which, in turn, is always after the SELECT keyword The italicized columnlist for the SELECT and ORDER BY keywords indicates that any number of columns can be listed The columns in

columnlist can be individual columns or more complex expressions The columns

specified after the SELECT and ORDER BY keywords can be entirely different

columns The italicized tablelist indicates that any number of tables can be listed,

although you have not yet seen the syntax for listing multiple tables

Turning to an example, you’ll be working from data in this Customers table:

CustomerID FirstName LastName

1 William Smith

2 Janet Smith

3 Natalie Lopez

4 Brenda Harper

Sorting in Ascending Order

If you want to sort data in alphabetic order, with A coming before Z, then you simply need to add anORDER BYclause to theSELECT For example:

SELECT

FirstName,

LastName

FROM Customers

ORDER BY LastName

brings back this data:

FirstName LastName

Brenda Harper

Natalie Lopez

William Smith

Janet Smith

Chapter 5 ■ Sorting Data

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Since there are two Smiths, William and Janet, there’s no way to predict which

one will be listed first This is because you are only sorting on LastName, and

there are multiple rows with the same last name

Similarly, if you issue thisSELECT:

SELECT

FirstName,

LastName

FROM Customers

ORDER BY FirstName

then this data is retrieved:

FirstName LastName

Brenda Harper

Janet Smith

Natalie Lopez

William Smith

The order is now completely different since you’re sorting by first name

SQL provides a special keyword namedASC, which stands for ascending This

keyword is completely optional and largely unnecessary since all sorts are

assumed to be in ascending order by default The followingSELECT, which uses

theASCkeyword, returns the same data as shown previously:

SELECT

FirstName,

LastName

FROM Customers

ORDER BY FirstName ASC

The keyword ASC is used to emphasize the fact that the sort is ascending, as

opposed to descending

Sorting in Descending Order

TheDESCkeyword sorts in an order opposite toASC Instead of ascending, the

order in such a sort is descending

For example:

SELECT

FirstName,

LastName

FROM Customers

ORDER BY FirstName DESC

retrieves:

FirstName LastName

William Smith

Natalie Lopez

Janet Smith

Brenda Harper

The first names are now in a Z to A order

Sorting by Multiple Columns

We now return to the problem of what to do with the Smiths If you want to sort

by last name, but there are two people with the same last name, you need to add a secondary sort by first name, as follows:

SELECT

FirstName,

LastName

FROM Customers

ORDER BY LastName, FirstName

This brings back:

FirstName LastName

Brenda Harper

Natalie Lopez

Janet Smith

William Smith

Since you are specifying a second sort column, you can now be certain that Janet Smith will appear before William Smith Note that theORDER BYclause needs to list LastName before FirstName The order of the columns is significant Your

Chapter 5 ■ Sorting Data

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