Beginning C# 2008 Databases From Novice to Professional phần 3 doc

To explicitly select columns, enter the col- umn names after the SELECTkeyword as shown in the following query and click Execute.. How It Works The new query returns the data for columns

Figure 5-2.Query results pane

How It Works

You ask the database to return the data for all columns, and you get exactly that If you

scroll to the right, you’ll find all the columns in the Employees table

Most of the time, you should limit queries to only relevant columns When you selectcolumns you don’t need, you waste resources To explicitly select columns, enter the col-

umn names after the SELECTkeyword as shown in the following query and click Execute

Figure 5-3 shows the results

Select employeeid, firstname, lastname

from employees

This query selects all the rows from the Employees table but only the EmployeeID,FirstName, and LastName columns

Figure 5-3.Selecting specific columns

Using the WHERE Clause

Queries can have WHEREclauses The WHEREclause allows you to specify criteria for ing rows This clause can be complex, but we’ll stick to a simple example for now Thesyntax is as follows:

select-WHERE <column1> <operator> <column2 / Value>

Here, <operator>is a comparison operator (for example, =,<>,>, or <) (Table 5-1, later inthe chapter, lists the T-SQL comparison operators.)

Try It Out: Refining Your Query

In this exercise, you’ll see how to refine your query

1. Add the following WHEREclause to the query in Figure 5-3

Where country = 'USA'

2. Run the query by pressing F5, and you should see the results shown in Figure 5-4

Figure 5-4.Using a WHEREclause

■ Caution SQL keywords and table and column names aren’t case sensitive, but string literals (enclosed

in single quotes) are This is why we use 'USA', not 'usa', for this example

How It Works

The new query returns the data for columns EmployeeID, FirstName, and LastName

from the Employees table, but only for rows where the Country column value equals

“USA”

Using Comparison Operators in a WHERE Clause

You can use a number of different comparison operators in a WHEREclause (see Table 5-1)

Table 5-1.Comparison Operators

< Less than EmployeeID < 1

> Greater than EmployeeID > 1

<= Less than or equal to EmployeeID <= 1

>= Greater than or equal to EmployeeID >= 1

<> Not equal to EmployeeID <> 1

!= Not equal to EmployeeID != 1

!< Not less than EmployeeID !< 1

!> Not greater than EmployeeID !> 1

■ Tip As mentioned earlier, every database vendor has its own implementation of SQL This discussion isspecific to T-SQL; for example, standard SQL doesn’t have the !=operator and calls <>the not equals oper- ator In fact, standard SQL calls the expressions in a WHEREclause 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 LIKEoperator (see Table 5-2) allows you to matchpatterns in character data As with all SQL character data, strings must be enclosed insingle quotes (') (Chapter 4 covers the LIKEoperator in more detail.)

Table 5-2.The LIKEOperator

LIKE Allows you to specify a pattern WHERE Title LIKE 'Sales%' selects all rows

where the Title column contains a value that starts with the word “Sales” followed by zero or more characters.

You can use four different wildcards in the pattern Chapter 4 covers these wildcards

in detail, but to briefly review, we list them here in Table 5-3

Table 5-3.Wildcard Characters

Wildcard Description

% Any combination of characters Where FirstName LIKE 'Mc%' selects all rows where

the FirstName column equals McDonald, McBadden, McMercy, and so on.

_ 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 bales or sales.

[^] A single character not within a range [^a-d] or set [^abcd].

Sometimes it’s useful to select rows where a value is unknown When no value hasbeen assigned to a column, the column is NULL (This isn’t the same as a column that con-

tains the value 0or a blank.) To select a row with a column that’sNULL, use the IS [NOT]

NULLoperator (see Table 5-4)

Table 5-4.The IS [NOT] NULLOperator

IS NULL Allows you to select rows where WHERE Region IS NULL returns all rows where

a column has no value Region has no value.

IS NOT NULL Allows you to select rows where WHERE Region IS NOT NULL returns all rows

a column has a value where Region has a value.

■ Note You must use the IS NULLand IS NOT NULLoperators (collectively called the null predicate in

standard SQL) to select or exclude NULLcolumn 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 read as SELECT * FROM employees WHERE Region IS NULL, and it will return rows where

regions have no value

To select values in a range or in a set, you can use the BETWEENand INoperators (seeTable 5-5) The negation of these two is NOT BETWEENand NOT IN

Table 5-5.The BETWEENand INOperators

BETWEEN True if a value is within a range WHERE extension BETWEEN 400 AND 500 returns

the rows where Extension is between 400 and

500, inclusive.

IN True if a value is in a list The list WHERE city IN ('Seattle', 'London') returns

can be the result of a subquery the rows where City is either Seattle or London.

Combining Predicates

Quite often you’ll need to use more than one predicate to filter your data You can use thelogical operators shown in Table 5-6

Table 5-6.SQL Logical Operators

AND Combines two expressions, HERE (title LIKE 'Sales%' AND lastname

evaluating the complete ='Peacock') expression as true only if both

are true NOT Negates a Boolean value WHERE NOT (title LIKE 'Sales%' AND lastname

='Peacock')

OR Combines two expressions, WHERE (title = 'Peacock' OR title = 'King')

evaluating the complete expression as true if either

is true

When you use these operators, it’s often a good idea to use parentheses to clarify theconditions In complex queries, this may 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 Since tables are by definition unsorted, the order in which rows areretrieved by a query is unpredictable To impose an ordering, you use the ORDER BYclause.ORDER BY <column> [ASC | DESC] {, n}

The <column>is the column that should be used to sort the result The {, n}syntaxmeans 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 ASCor DESCkeywords, the sort order defaults to ASC.The following is the basic syntax for queries:

SELECT <column>

FROM <table>

WHERE <predicate>

ORDER BY <column> ASC | DESC

Now that you’ve seen it, you’ll put this syntax to use in an example

Try It Out: Writing an Enhanced Query

In this example, you’ll code a query that uses the basic syntax just shown You want to do

the following:

• Select all the orders that have been handled by employee 5

• Select the orders shipped to either France or Brazil

• Display only OrderID, EmployeeID, CustomerID, OrderDate, and ShipCountry

• Sort the orders by the destination country and the date the order was placed

Does this sound complicated? Give it a try Open a New Query window in SQL ServerManagement Studio Enter the following query and click Execute You should see the

results shown in Figure 5-5

select orderid,employeeid,customerid,orderdate,shipcountry

from orders

where employeeid = 5 and shipcountry in ('Brazil', 'France')

order by shipcountry asc,orderdate asc

Figure 5-5.Filtering and sorting data

As these predicates are combined with AND, they both must evaluate to true for a row

to be included in the result

where employeeid = 5 and shipcountry in ('Brazil', 'France')

The ORDER BYclause specifies the order in which the rows are sorted The rows will besorted by ShipCountry first and then by OrderDate

order by shipcountry asc,orderdate asc

Using SELECT INTO Statements

ASELECT INTOstatement is used to create a new table containing or not containing the

result set returned by a SELECTquery SELECT INTOcopies the exact table structure and

data into another table specified in the INTOclause Usually, a SELECTquery returns result

sets to the client application

Including the #(hash) symbol before table name results in creating a temporarytable, which ends up in the tempdb system database, regardless of which database you

are working in Specifying the table name without the #symbol gives you a permanent

table in your database (not in tempdb)

The columns of the newly created table inherit the column names, their data types,whether columns can contain null values or not, and any associated IDENTITYproperty

from the source table However, the SELECT INTOclause does have some restrictions: it

will not copy any constraints, indexes, or triggers from the source table

Try It Out: Creating a New Table

In this exercise, you’ll see how to create a table using a SELECT INTOstatement Open a

New Query window in SQL Server Management Studio Express (remember to make

Northwind your query context) Enter the following query and click Execute You should

see the results shown in Figure 5-6

select orderid,employeeid,customerid,orderdate,shipcountry

into #myorder

from orders

Figure 5-6.Creating a new table

Even though you write the query in Northwind, the #myorder table gets createdinside tempdb because of the prefixed #symbol (see Figure 5-7)

A temporary table can reside in the tempdb database as long as you have the querywindow open If you close the query window from which you created your temporarytable, and regardless of whether you saved the query, the temporary table will be auto-matically deleted from tempdb

Once the table is created, you can use it like any other table (see Figure 5-8).Temporary tables will also be deleted if you close SQL Server Management StudioExpress, because the tempdb database gets rebuilt every time you close and open SQLServer Management Studio Express again

Figure 5-7.Viewing the newly created table in tempdb

Figure 5-8.Retrieving data from your temporary table

Try It Out: Using SELECT INTO to Copy Table Structure

Sometimes you will want to copy only the table structure, not the data inside the table(e.g., you only need an empty copy of the table) To do so, you need to include a conditionthat must not return true In this case, you are free to insert your own data

To try this out, enter the following query, and you should get the results shown inFigure 5-9

it, and select Refresh to refresh the tables list You should see the newly created

#myemptyorder table as shown in Figure 5-10

Figure 5-10.Viewing a newly created empty table in tempdb

As you can see, the table has structure but not data, the false condition you included

If you were to run a SELECTquery on the #myemptyorder table as shown in Figure 5-11, the query would return nothing, clearly demonstrating that only the

table structure has been copied because only field names are displayed

Figure 5-11.Writing a SELECTquery on an empty table in tempdb

Inserting Data

The next important task you need to be able to do is add data (e.g., add rows) to a table.You do this with the INSERTstatement The INSERTstatement is much simpler than aquery, particularly because the WHEREand ORDER BYclauses have no meaning when insert-ing data and therefore aren’t used

A basic INSERTstatement has these parts:

INSERT INTO <table>

(<column1>, <column2>, , <columnN>)

VALUES (<value1>, <value2>, , <valueN>)

Using this syntax, let’s add a new row to the Shipperstable of the Northwind base Before you insert it, let’s look at the table In the SQL Server Management StudioExpress Object Explorer, select the Northwind database, right-click the Shippers table,and click Open Table The table has three rows, which are displayed in a tabbed window(see Figure 5-12)

data-Figure 5-12.The Shippers table before adding a row

Try It Out: Inserting a New Row

To insert a new row into a table, open a New Query window in SQL Server ManagementStudio Express Enter the following query and click Execute

insert into shippers ( companyname, phone )

The first column, ShipperID, is an identity column, and you can’t insert values into it

explicitly—SQL Server database engine will make sure that a unique and SQL server–

generated value is inserted for the ShipperID field So, the INSERTstatement needs to be

written in such a way that you specify the column list you want to insert values for

explic-itly; though the Shippers table contains three fields, ShipperID is an identity column, and

it does not expect any value to be inserted from the user But by default, an INSERT

state-ment cannot judge whether the column you are not passing a value for is an identity

column Thus, to prevent errors, you specify the column list and then pass the respective

values to these fields as shown in the following query:

insert into shippers( companyname, phone )

values ('GUIPundits', '+91 9820801756')

■ Note INSERTstatements have a limitation When you try to insert data directly into a foreign key table,and the primary key table has no related parent record, you will receive an error because that value needs to

be available in the primary key table before you insert it into the foreign key table For example, the Shipperstable is the PK table for the Orders table, which has an FK column named ShipVia that references the PK col-umn ShipperID of Shippers table In this scenario, you can’t insert a row until you have inserted it into theShippers table

After inserting the row, return to the dbo.Shippers table in Object Explorer, click, and open the table again You’ll see that the new row has been added, as shown inFigure 5-14

right-Figure 5-14.The Shippers table after adding a row

Be careful to insert data of the correct data type In this example, both the columnsare of character type, so you inserted strings If one of the columns had been of integertype, you would have inserted an integer value instead

Updating Data

Another important task you need to be able to do is change data You do this with the

UPDATEstatement When coding UPDATEstatements, you must be careful to include a WHERE

clause, or you’ll update all the rows in a table So, always code an appropriate WHERE

clause, or you won’t change the data you intend to change

Now that you’re aware of the implications of the UPDATEstatement, let’s take a goodlook at it In essence, it’s a simple statement that allows you to update values in one or

more rows and columns

changing its name to Pearl HR Solution To make this change in the database, you first

need to locate the row to change More than one company could have the same name,

so you shouldn’t use the CompanyName column as the key Instead, look back at

Figure 5-10 and note the ShipperID value for GUIPundits

Try It Out: Updating a Row

To change a row’s value, open a New Query window in SQL Server Management Studio

Express Enter the following query and click Execute

update shippers

set companyname = 'PearlHRSolution'

where shipperid = 4

How It Works

The ShipperID is the primary key (unique identifier for rows) of the Shippers table, so

you can use it to locate the one row we want to update Running the query should

pro-duce a Messages pane reporting “(1 row(s) affected)” Switch back to Object Explorer and

open the Shippers table, and you’ll see that CompanyName has changed, as shown in

Figure 5-15

Figure 5-15.The Shippers table after updating a row

When you update more than one column, you still use the SETkeyword only once,and separate column names and their respective values you want to set by comma Forexample, the following statement would change both the name and the phone of thecompany:

Figure 5-16.The Shippers table after updating multiple columns of a row

Deleting Data

The final important task you need to be able to do that we’ll discuss in this chapter is

remove data You do this with the DELETEstatement The DELETEstatement has the same

implications as the UPDATEstatement It’s all too easy to delete every row (not just the wrong

rows) in a table by forgetting the WHEREclause, so be careful The DELETEstatement removes

entire rows, so it’s not necessary (or possible) to specify columns Its basic syntax is as

fol-lows (remember, the WHEREclause is optional, but without it all rows will be deleted):

DELETE FROM <table>

Figure 5-17.The Shippers table after deleting a row

If you try to delete one of the remaining three shippers, you’ll get a database error Aforeign-key relationship exists from Orders (FK) to Shippers (PK), and SSE enforces it,preventing deletion of Shippers’ rows that are referred to by Orders rows If the databasewere to allow you to drop records from the PK table, the records in the FK table would beleft as orphan records, leaving the database in an inconsistent state (Chapter 3 discusseskeys.)

Sometimes you do need to remove every row from a table In such cases, the TRUNCATETABLEstatement may be preferable to the DELETEstatement, since it performs better TheTRUNCATE TABLEstatement is faster because it doesn’t do any logging (saving each row in a

log file before deleting it) to support recovery, while DELETElogs every row removed

Summary

In this chapter, you saw how to use the following T-SQL keywords to perform data ulation tasks against a database: SELECT INTO,SELECT,INSERT,UPDATE, and DELETE You alsosaw how to use comparison and other operators to specify predicates that limit whatrows are retrieved or manipulated

manip-In the next chapter, you will see how stored procedures work

Using Stored Procedures

Stored procedures are SQL statements that allow you to perform a task repeatedly.

You can create a procedure once and reuse it any number of times in your program

This can improve the maintainability of your application and allow applications to

access the database in a uniform and optimized manner The goal of this chapter is to

get you acquainted with stored procedures and understand how C# programs can

interact with them

In this chapter, we’ll cover the following:

• Creating stored procedures

• Modifying stored procedures

• Displaying definitions of stored procedures

• Renaming stored procedures

• Working with stored procedures in C#

• Deleting stored procedures

Creating Stored Procedures

Stored procedures can have parameters that can be used for input or output and

single-integer return values (that default to zero), and they can return zero or more result sets.

They can be called from client programs or other stored procedures Because stored

procedures are so powerful, they are becoming the preferred mode for much database

programming, particularly for multitier applications and web services, since (among

their many benefits) they can dramatically reduce network traffic between clients and

database servers

95

C H A P T E R 6

Try It Out: Working with a Stored Procedure in SQL Server

Using SQL Server Management Studio Express, you’ll create a stored procedure that duces a list of the names of employees in the Northwind database It requires no inputand doesn’t need to set a return value

pro-1. Open SQL Server Management Studio Express, and in the Connect to Server log box, select <ServerName>\SQLEXPRESS as the server name and then clickConnect

dia-2. In Object Explorer, expand the Databases node, select the Northwind database,and click the New Query button Enter the following query and click Execute Youshould see the results shown in Figure 6-1

create procedure sp_Select_All_Employeesas

selectemployeeid,firstname,lastnamefromemployees

Figure 6-1.Creating a stored procedure using SQL Server Management Studio Express

3. To execute the stored procedure, enter the following query and click Execute Youshould see the results shown in Figure 6-2.

execute sp_Select_All_Employees

Figure 6-2.Executing the stored procedure

How It Works

The CREATE PROCEDUREstatement creates stored procedures The ASkeyword separates the

signature (the procedure’s name and parameter list, but here you define no parameters)

of the stored procedure from its body (the SQL that makes up the procedure)

create procedure sp_Select_All_Employees

as

After AS, the procedure body has just one component, a simple query

Select

employeeid,firstname,lastnamefrom

employees

SQL Server Management Studio Express submitted the CREATE PROCEDUREstatement,and once the stored procedure is created, you run it from the query window by writingthe statement

execute sp_Select_All_Employees

That’s it There’s nothing complicated about creating stored procedures The lenge is coding them when they’re nontrivial, and stored procedures can be quite compli-cated and can do very powerful things, but that’s well beyond the scope of this book

chal-■ Note The prefix sp_is a T-SQL convention that typically indicates the stored procedure is coded in SQL.The prefix xp_(which stands for extended procedure) is also used to indicate that the stored procedure isn’twritten in SQL (However, not all sp_stored procedures provided by SQL Server are written in SQL.) By theway, hundreds of sp_(and other) stored procedures are provided by SQL Server 2005 to perform a widevariety of common tasks

Although we use sp_for the purposes of these examples, it is a best practice not tocreate a stored procedure prefixed with sp_; doing so has a dramatic effect on the searchmechanism and the way the SQL Server database engine starts searching for that partic-ular procedure in order to execute

The SQL Server follows this search order if you are executing a stored procedure thatbegins with sp_:

1. SQL Server will search the master database for the existence of the procedure, if it

is available, and then it will call the procedure

2. If the stored procedure is not available in the master database, SQL Serversearches inside either the database from which you are calling it or the databasewhose name you provide as qualifier (database_name.stored_procedure_name).Therefore, although a user-created stored procedure prefixed with sp_may exist in

the current database, the master database, which is where the sp_prefixed stored dures that come with SQL Server 2005 are stored, is always checked first, even if thestored procedure is qualified with the database name

proce-It is also important to note that if any user-defined stored procedure has the samename as a system stored procedure, and you try calling the user-defined stored proce-dure, it will never be executed, even if you call it from inside the database where you havejust created it Only the master database’s version will be called

Try It Out: Creating a Stored Procedure with an Input Parameter

Here you’ll create a stored procedure that produces a list of orders for a given employee

You’ll pass the employee ID to the stored procedure for use in a query

1. Enter the following query and click Execute You should see the message mand(s) completed successfully” in the results window

“Com-create procedure sp_Orders_By_EmployeeId

@employeeid intas

select orderid, customeridfrom orders

where employeeid = @employeeid;

2. To execute the stored procedure, enter the following command along with thevalue for the parameter, select it, and then click Execute You should see theresults shown in Figure 6-3

execute sp_Orders_By_EmployeeId 2

Figure 6-3.Using an input parameter

■ Tip SQL Server has a very interesting behavior of executing a portion of a query or executing a particularquery out of multiple SQL statements written in the query window, unlike other RDBMSs This behavior isshown in the Figure 6-3, in which we have selected a particular statement Click the Execute button, andSQL Server will process only the selected statement.

How It Works

The CREATE PROCEDUREstatement creates a stored procedure that has one input parameter.Parameters are specified between the procedure name and the ASkeyword Here youspecify only the parameter name and data type, so by default it is an input parameter.Parameter names start with @

create procedure sp_Orders_By_EmployeeId

@employeeid intas

This parameter is used in the WHEREclause of the query

where

employeeid = @employeeid;

■ Note In this example, a semicolon terminates the query It’s optional here, but you’ll see when it needs to

be used in the next example

Try It Out: Creating a Stored Procedure with an

Output Parameter

Output parameters are usually used to pass values between stored procedures, butsometimes they need to be accessed from C#, so here you’ll see how to write a storedprocedure with an output parameter so you can use it in a C# program later You’ll alsosee how to return a value other than zero