It is often necessary to make the query text from constant pieces and variables.
For example we have a many tables with such structure:
(
ID INT IDENTITY(1,1) PRIMARY KEY,
Number INT,
ChangeDate DATETIME
)
and we need dynamically, depending on variables with table name, [Number] min value and [ChangeDate] min value send queries to one or other table.
We can do it using Dynamic SQL method, for example so:
CREATE PROCEDURE MySP
@ObjectName SYSNAME,
@MinNumber INT,
@ValidFrom DATETIME
AS BEGIN
DECLARE @SQL NVarChar(MAX);
SET @SQL = '
SELECT
*
FROM
' + @ObjectName + '
WHERE
Number >= ' + CAST(@MinNumber AS VARCHAR(20)) + '
AND
ChangeDate >= CONVERT(DATETIME, '''
+ CONVERT(VARCHAR(30), @ValidFrom, 121) + ''', 121);';
PRINT @SQL;
EXEC sp_executesql @SQL;
END;
GO
EXEC MySP 'MyTableNr1', 123, '2015-01-01 12:00';
It works, but SQL-Text format is not readable.
Much more readable looks the text converted to a template with placeholders:
'
SELECT
*
FROM
{ObjectName}
WHERE
Number >= {MinNumber}
AND
ChangeDate >= CONVERT(DATETIME, ''{ValidFrom}'';
'
In this case our example stored procedure will look like this:
CREATE PROCEDURE MySP
@ObjectName SYSNAME,
@MinNumber INT,
@ValidFrom DATETIME
AS BEGIN
DECLARE @SQL NVarChar(MAX);
SET @SQL = '
SELECT
*
FROM
{ObjectName}
WHERE
Number >= {MinNumber}
AND
ChangeDate >= CONVERT(DATETIME, ''{ValidFrom}'';'
SET @SQL = REPLACE(@SQL, '{ObjectName}', @ObjectName);
SET @SQL = REPLACE(@SQL, '{MinNumber}', CAST(@MinNumber AS VARCHAR(20)));
SET @SQL = REPLACE(@SQL, '{ValidFrom}', CONVERT(VARCHAR(30), @ValidFrom, 121));
PRINT @SQL;
EXEC sp_executesql @SQL;
END;
It is good for so simple example, but if we will have more dynamical parameters and inside one stored procedure (or plain query) more then one template, then this query will have too many redundant call like this:
SET @SQL = REPLACE(@SQL, ..., ...);
The solution is to create some help procedure which will take text template and dictionary with placeholder
name:value pairs as parameters.
Stored procedure can not take table-value parameters directly.
Table-valued parameters
have to be declared by using user-defined table types.
In our case:
CREATE TYPE PlaceholderMappingType AS TABLE (
Placeholder NVARCHAR(100),
Value NVARCHAR(MAX)
);
The complete script with usage example:
USE tools;
GO
IF OBJECT_ID('tools.dbo.sp_ReplacePlaceholders') IS NOT NULL BEGIN
PRINT '-- DROP PROCEDURE dbo.sp_ReplacePlaceholders';
DROP PROCEDURE dbo.sp_ReplacePlaceholders;
END;
GO
IF EXISTS (
SELECT *
FROM tools.INFORMATION_SCHEMA.DOMAINS
WHERE DOMAIN_NAME = 'PlaceholderMappingType'
) BEGIN
PRINT '-- DROP TYPE dbo.PlaceholderMappingType';
DROP TYPE dbo.PlaceholderMappingType;
END;
GO
PRINT '-- CREATE TYPE dbo.PlaceholderMappingType';
-- TABLE-Domain (User Defined Type) for placeholders dictionary
CREATE TYPE PlaceholderMappingType AS TABLE (
Placeholder NVARCHAR(100),
Value NVARCHAR(MAX)
);
GO
CREATE PROCEDURE dbo.sp_ReplacePlaceholders (
@Text NVARCHAR(MAX) OUTPUT,
-- TABLE-Domain (User Defined Type) for placeholders dictionary
@PlaceholderMappings PlaceholderMappingType READONLY,
-- Flag to replace in placeholder values each single apostrof with double apostrof
@EscapeApostrofs BIT = 1,
-- Text to TrimLeft in each text line
@IndentString VARCHAR(MAX) = '',
-- Flag to PRINT additional diagnostic information
@Debug BIT = 0
)
AS BEGIN
SET NOCOUNT ON;
IF (@Debug = 1)
PRINT '-- EXEC dbo.sp_ReplacePlaceholders'
DECLARE
@CRLF CHAR(2) = CHAR(13)+CHAR(10),
@Indent_CharIndex INT,
@I INT = 0,
@Char CHAR(1),
@Indent VARCHAR(MAX) = '';
-- Trim rows left
IF (CHARINDEX(@IndentString, @Text, 0) = 1)
SET @Text = STUFF(@Text, 1, LEN(@IndentString), '');
SET @Text = REPLACE(@Text, @CRLF + @IndentString, @CRLF);
-- Declare FOREACH(placeholderRow IN @PlaceholderMappings) cursor
DECLARE
@Placeholder NVARCHAR(102),
@Value NVARCHAR(MAX);
DECLARE Placeholders_Cursor CURSOR
LOCAL STATIC READ_ONLY FORWARD_ONLY
FOR
SELECT
Placeholder = '{' + REPLACE(REPLACE(Placeholder, '{', ''), '}', '') + '}',
Value = CASE @EscapeApostrofs
WHEN 1
THEN REPLACE(Value, '''', '''''')
ELSE Value
END
FROM @PlaceholderMappings;
OPEN Placeholders_Cursor;
FETCH NEXT FROM Placeholders_Cursor INTO @Placeholder, @Value;
WHILE @@FETCH_STATUS = 0 BEGIN
IF (@Debug = 1) AND (CHARINDEX(@Placeholder, @Text) = 0)
PRINT '-- ==> Could not find Placeholder ' + @Placeholder;
SET @Text = REPLACE(@Text, @Placeholder, @Value);
FETCH NEXT FROM Placeholders_Cursor INTO @Placeholder, @Value;
END;
CLOSE Placeholders_Cursor
DEALLOCATE Placeholders_Cursor
RETURN;
END;
GO
DECLARE
@SQL NVARCHAR(MAX),
@PlaceholderMappings PlaceholderMappingType;
SET @SQL = '
SELECT
F1=''{aaa}'',
F2=''{bbb}'',
F3=''{ddd}'';';
INSERT INTO @PlaceholderMappings
VALUES
('aaa', 'A''A''A'),
('{bbb}', 'BBB'),
('ccc', 'CCC');
DECLARE @IndentString VARCHAR(MAX) = SPACE(8);
EXEC dbo.sp_ReplacePlaceholders
@SQL OUTPUT,
@PlaceholderMappings,
@IndentString=@IndentString,
@Debug=1;
PRINT '>>>' + @SQL + '<<<';
