sql server - How can I replace all key fields in a string with replacement values from a table in T-SQL?

Question

I have a table like:

 TemplateBody
 ---------------------------------------------------------------------
 1.This is To inform #FirstName# about the issues regarding #Location#

Here the key strings are #FirstName# and #Location# which are distinguished by hash tags.

I have another table with the replacement values:

Variables     | TemplateValues
-----------------------------
1.#FirstName# | Joseph William
2.#Location#  | Alaska

I need to replace these two key strings with their values in the first table.

Answer 1

There are several ways this can be done. I'll list two ways. Each one has advantages and disadvantages. I would personally use the first one (Dynamic SQL).

1. Dynamic SQL

• Advantages: Fast, doesn't require recursion
• Disadvantages: Can't be used to update table variables

2. Recursive CTE

• Advantages: Allows updates of table variables
• Disadvantages: Requires recursion and is memory intensive, recursive CTE's are slow

---

1.A. Dynamic SQL: Regular tables and Temporary tables.

This example uses a temporary table as the text source:

CREATE TABLE #tt_text(templatebody VARCHAR(MAX));
INSERT INTO #tt_text(templatebody)VALUES
    ('This is to inform #first_name# about the issues regarding #location#');

CREATE TABLE #tt_repl(variable VARCHAR(256),template_value VARCHAR(8000));
INSERT INTO #tt_repl(variable,template_value)VALUES
    ('#first_name#','Joseph William'),
    ('#location#','Alaska');

DECLARE @rep_call NVARCHAR(MAX)='templatebody';
SELECT
    @rep_call='REPLACE('+@rep_call+','''+REPLACE(variable,'''','''''')+''','''+REPLACE(template_value,'''','''''')+''')'
FROM
    #tt_repl;

DECLARE @stmt NVARCHAR(MAX)='SELECT '+@rep_call+' FROM #tt_text';
EXEC sp_executesql @stmt;

/* Use these statements if you want to UPDATE the source rather than SELECT from it
DECLARE @stmt NVARCHAR(MAX)='UPDATE #tt_text SET templatebody='+@rep_call;
EXEC sp_executesql @stmt;
SELECT * FROM #tt_text;*/

DROP TABLE #tt_repl;
DROP TABLE #tt_text;

---

1.B. Dynamic SQL: Table variables.

Requires to have the table defined as a specific table type. Example type definition:

CREATE TYPE dbo.TEXT_TABLE AS TABLE(
    id INT IDENTITY(1,1) PRIMARY KEY,
    templatebody VARCHAR(MAX)
);
GO

Define a table variable of this type, and use it in a Dynamic SQL statement as follows. Note that updating a table variable this way is not possible.

DECLARE @tt_text dbo.TEXT_TABLE;
INSERT INTO @tt_text(templatebody)VALUES
    ('This is to inform #first_name# about the issues regarding #location#');

DECLARE @tt_repl TABLE(id INT IDENTITY(1,1),variable VARCHAR(256),template_value VARCHAR(8000));
INSERT INTO @tt_repl(variable,template_value)VALUES
    ('#first_name#','Joseph William'),
    ('#location#','Alaska');

DECLARE @rep_call NVARCHAR(MAX)='templatebody';
SELECT
    @rep_call='REPLACE('+@rep_call+','''+REPLACE(variable,'''','''''')+''','''+REPLACE(template_value,'''','''''')+''')'
FROM
    @tt_repl;

DECLARE @stmt NVARCHAR(MAX)='SELECT '+@rep_call+' FROM @tt_text';
EXEC sp_executesql @stmt,N'@tt_text TEXT_TABLE READONLY',@tt_text;

---

2. Recursive CTE:

The only reasons why you would write this using a recursive CTE is that you intend to update a table variable, or you are not allowed to use Dynamic SQL somehow (eg company policy?).

Note that the default maximum recursion level is 100. If you have more than a 100 replacement variables you should increase this level by adding OPTION(MAXRECURSION 32767) at the end of the query (see Query Hints - MAXRECURSION).

DECLARE @tt_text TABLE(id INT IDENTITY(1,1),templatebody VARCHAR(MAX));
INSERT INTO @tt_text(templatebody)VALUES
    ('This is to inform #first_name# about the issues regarding #location#');

DECLARE @tt_repl TABLE(id INT IDENTITY(1,1),variable VARCHAR(256),template_value VARCHAR(8000));
INSERT INTO @tt_repl(variable,template_value)VALUES
    ('#first_name#','Joseph William'),
    ('#location#','Alaska');

;WITH cte AS (
    SELECT
        t.id,
        l=1,
        templatebody=REPLACE(t.templatebody,r.variable,r.template_value)
    FROM
        @tt_text AS t
        INNER JOIN @tt_repl AS r ON r.id=1
    UNION ALL
    SELECT
        t.id,
        l=l+1,
        templatebody=REPLACE(t.templatebody,r.variable,r.template_value)
    FROM
        cte AS t
        INNER JOIN @tt_repl AS r ON r.id=t.l+1
)
UPDATE
    @tt_text
SET
    templatebody=cte.templatebody
FROM
    @tt_text AS t
    INNER JOIN cte ON 
        cte.id=t.id
WHERE
    cte.l=(SELECT MAX(id) FROM @tt_repl);

/* -- if instead you wanted to select the replaced strings, comment out 
   -- the above UPDATE statement, and uncomment this SELECT statement:
SELECT 
    templatebody 
FROM 
    cte 
WHERE 
    l=(SELECT MAX(id) FROM @tt_repl);*/

SELECT*FROM @tt_text;