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Tables, Data Types, and Declarative Data Integrity
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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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// C# [SqlFunction(IsDeterministic = true, DataAccess=DataAccessKind.None)] static public SqlBoolean fnIsPhoneNumber(SqlString phoneNumber) { if (phoneNumber.IsNull){ return SqlBoolean.Null; } return System.Text.RegularExpressions.Regex .IsMatch(phoneNumber.Value, @"^\([1-9]\d{2}\)\s \d{3}\-\d{4}$"); }
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UDFs are explained in detail in 7, Extending Microsoft SQL Server Functionality with XML, SQLCLR, and Filestream.
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The following code creates a table and the check constraint that references the UDF:
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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)) );
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-- Allowed: INSERT Test.Contacts (Name, PhoneNumber) VALUES ('Tobias', '(425)555-1111'); INSERT Test.Contacts (Name, PhoneNumber) VALUES ('Chris', NULL);
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-- Disallowed, will raise an error: INSERT Test.Contacts (Name, PhoneNumber) VALUES ('Ann', '(42)555-2222');
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Lesson 2: Declarative Data Integrity
When is this check constraint executed Only when needed. The optimizer runs 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 executed only if a column contained in the check constraint is referenced by the update.
note
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.
USING A UDF WITH A SUBQUERY
It is possible to include subqueries in check constraints by placing them inside a UDF. This practice can result in poor performance because the subquery is executed once for each row affected by an UPDATE or INSERT statement against the table. Imagine you want to extend the previous example to also validate the telephone area codes using a subquery. The supported area codes are stored in a separate table called Test.AreaCodes. Here is the extended version of the UDF:
VB <Microsoft.SqlServer.Server.SqlFunction(IsDeterministic:=True, _ DataAccess:=DataAccessKind.Read)> _ Public Shared Function fnIsPhoneNumber2(ByVal phoneNumber As SqlString) _ As SqlBoolean
If (phoneNumber.IsNull) Then Return SqlBoolean.Null End If
If Not System.Text.RegularExpressions.Regex.IsMatch(phoneNumber.Value, _ "^\([1-9]\d{2}\)\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"
Tables, Data Types, and Declarative Data Integrity
cmd.Parameters.Add("@AreaCode", SqlDbType.Char, 3).Value = areaCode conn.Open() Return CType(cmd.ExecuteScalar(), Boolean) End Using End Using End If End Function
// C# [SqlFunction(IsDeterministic = true, DataAccess=DataAccessKind.Read)] static public SqlBoolean fnIsPhoneNumber2(SqlString phoneNumber) { if(phoneNumber.IsNull) return SqlBoolean.Null;
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(); } } } }
The following code creates a table and the check constraint referencing the UDF:
CREATE TABLE Test.AreaCodes ( AreaCode CHAR(3) NOT NULL PRIMARY KEY ); -- The only allowed area code. INSERT Test.AreaCodes (AreaCode) VALUES ('425');
Lesson 2: Declarative Data Integrity
CREATE TABLE Test.Contacts ( ContactID INT IDENTITY PRIMARY KEY ,Name NVARCHAR(50) NOT NULL ,PhoneNumber VARCHAR(20) NULL ,CONSTRAINT CKContactsPhoneNumber CHECK(dbo.fnIsPhoneNumber2(PhoneNumber) = CAST(1 AS BIT)) );
-- Allowed: INSERT Test.Contacts (Name, PhoneNumber) VALUES ('Ann', '(425)555-1111'); INSERT Test.Contacts (Name, PhoneNumber) VALUES ('Chris', NULL);
-- Disallowed because of invalid area code: INSERT Test.Contacts (Name, PhoneNumber) VALUES ('Tobias', '(111)555-2222');
A very imporant consideration when using subqueries in check constraints is that, while the check constraint is verified for UPDATEs and INSERTs to the table, it is not verified when deleting rows in the table that the subquery references. The data that the check constraint validated against on the INSERT or UPDATE can be deleted without raising an error. For example, the following DELETE statement does not result in an error:
DELETE Test.AreaCodes WHERE AreaCode = '425';
However, after executing the DELETE statement, the following UPDATE statement raises an error:
UPDATE Test.Contacts SET PhoneNumber = PhoneNumber;
This behavior is highly undesirable because you might think you have the same protection that you have with foreign keys, which protect you against the DELETE statement as well. In SQL Server 2008 (and 2005), you can often replace this logic by using a foreign key, as described in the next section.
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