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Lesson 1: Validating Data and Permissions
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.IsMatch(phoneNumber.Value, @"^\([1-9]\d{2}\)\s \d{3}\-\d{4}$"); } 'VB <Microsoft.SqlServer.Server.SqlFunction(IsDeterministic:=True, DataAccess:=DataAccessKind.None)> _ Public Shared Function fnIsPhoneNumber(ByVal phoneNumber As SqlString) As SqlBoolean If (phoneNumber.IsNull) Then Return SqlBoolean.Null End If Return System.Text.RegularExpressions.Regex.IsMatch(phoneNumber.Value, "^\([1-9]\d{2}\)\s \d{3}\-\d{4}$") End Function
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The following T-SQL statement creates a table and check constraint that references the userdefined function:
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CREATE TABLE Test.Contacts ( ContactID INT IDENTITY PRIMARY KEY ,Name NVARCHAR(50) NOT NULL ,PhoneNumber VARCHAR(20) NULL ,CONSTRAINT CKContactsPhoneNumber CHECK(dbo.fnIsPhoneNumber(PhoneNumber) = CAST(1 AS BIT)) ); -- Allowed: INSERT Test.Contacts (Name, PhoneNumber) VALUES ('Tobias', '(425)555-1111'); INSERT Test.Contacts (Name, PhoneNumber) VALUES ('Sara', NULL); -- Disallowed, will raise an error: INSERT Test.Contacts (Name, PhoneNumber) VALUES ('Bill', '(42)555-2222');
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When is the check constraint executed Only when needed. The query optimizer will decide to run the check constraint only if columns referenced in the check constraints are referenced by the executed DML statement. For inserts, this is always true because an insert always affects all columns (even if you insert a null value). For updates, the check constraint is only executed if a column contained in the check constraint is referenced by the update. If you turn on the Actual Execution Plan in SQL Server Management Studio and run the two update statements in the following code sample, you will get the execution plan shown in Figure 5-1. Notice the Assert icon in Query 2 of the execution plan; this is the execution of the check constraint. It is not executed for Query 1.
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UPDATE Test.Contacts SET Name = Name; UPDATE Test.Contacts SET PhoneNumber = PhoneNumber;
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Data Integrity and Error Handling in SQL Server 2005
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Figure 5-1 Actual Execution Plan in SQL Server Management Studio
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Performance
Adding a lot of logic to your check constraints can hurt performance. A good approach is to add the necessary constraints and then run a performance test to verify that the performance is sufficent.
Here you can see a very simple test that can be used to indicate whether you have the constraint in place:
DECLARE @i INT; SET @i = 1; WHILE(@i < 100000) BEGIN UPDATE Test.Contacts SET PhoneNumber = PhoneNumber; SET @i = @i + 1; END
In one test, this sample script took about six seconds to run with the constraint in place. Remember that there are only two rows in the table. By turning off the check constraint (ALTER TABLE Test.Contacts NOCHECK CONSTRAINT CKContactsPhoneNumber;), the same batch took about three seconds to run, which is half the length of time. Using a User-Defined Function with a Subquery It is possible to include subqueries in check constraints by putting them inside a user-defined function. This pratice can result in poor performance because the subquery will be executed once for each row affected by an update or insert statement against the table. Using the previous example, you can also validate telephone area codes using a subquery. Store the allowed area codes in a separate table. The following code sample is a user-defined function:
//C# [SqlFunction(IsDeterministic = true, DataAccess=DataAccessKind.Read)] static public SqlBoolean fnIsPhoneNumber(SqlString phoneNumber) { if(phoneNumber.IsNull) return SqlBoolean.Null;
Lesson 1: Validating Data and Permissions
if(!System.Text.RegularExpressions.Regex .IsMatch(phoneNumber.Value, @"^\([1-9]\d{2}\)\s \d{3}\-\d{4}$")){ return false; }else{ string areaCode = phoneNumber.Value.Substring(1,3); using(SqlConnection conn = new SqlConnection(@"context connection=true;")) { using(SqlCommand cmd = conn.CreateCommand()) { cmd.CommandText = @"IF EXISTS(SELECT * FROM Test.AreaCodes WHERE AreaCode = @AreaCode) SELECT CAST(1 AS BIT) AS Found ELSE SELECT CAST(0 AS BIT) AS Found"; cmd.Parameters.Add("@AreaCode", SqlDbType.Char, 3) .Value = areaCode; conn.Open(); return (bool)cmd.ExecuteScalar(); } } } } 'VB <Microsoft.SqlServer.Server.SqlFunction(IsDeterministic:=True, DataAccess:=DataAccessKind.None)> _ Public Shared Function fnIsPhoneNumber(ByVal phoneNumber As SqlString) As SqlBoolean If (phoneNumber.IsNull) Then Return SqlBoolean.Null End If If Not System.Text.RegularExpressions.Regex.IsMatch(phoneNumber.Value, "^\([19]\d{2}\)\s \d{3}\-\d{4}$") Then Return False Else Dim areaCode As String = phoneNumber.Value.Substring(1, 3) Using conn As SqlConnection = New SqlConnection("context connection=true;") Using cmd As SqlCommand = conn.CreateCommand() cmd.CommandText = "IF EXISTS(SELECT * FROM Test.AreaCodes " & _ "WHERE AreaCode = @AreaCode) " & _ "SELECT CAST(1 AS BIT) AS Found " & _ "ELSE " & _ "SELECT CAST(0 AS BIT) AS Found" cmd.Parameters.Add("@AreaCode", SqlDbType.Char, 3).Value = areaCode conn.Open() Return CType(cmd.ExecuteScalar(), Boolean
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