Inserting Rows from One Table into Different Tables

Introduction

In this article, we will discuss a common problem in data migration and integration: inserting rows from one table into different tables with varying column definitions. We will explore two approaches to solve this issue using dynamic SQL.

The Problem

Given a single-column table with string rows and columns delimited by pipes (|), we need to insert these rows into four different tables, each with its own unique column definition. The tables are identified by the first two characters of the row identifier. For example, if the row has “aa” as the identifier, it will be inserted into table aa.

The challenge arises when dealing with varying table definitions and field lengths. In this article, we will delve into the details of creating a dynamic SQL solution to address this issue.

Approach 1: Using Dynamic SQL with CASE Statements

One way to solve this problem is by using dynamic SQL with CASE statements. The idea is to create a stored procedure that takes as input the row identifier and returns a string containing the dynamic SQL query to insert the row into the corresponding table.

Here’s an example of how this approach could be implemented:

CREATE PROCEDURE dbo.IngestRows
(
  @InputTable VARCHAR(255),
  @OutputTable1 VARCHAR(255),
  @OutputTable2 VARCHAR(255),
  @OutputTable3 VARCHAR(255),
  @OutputTable4 VARCHAR(255)
)
AS
BEGIN
  DECLARE @Row VARCHAR(255);
  DECLARE @Count INT;
  DECLARE @MaxCount INT = (SELECT COUNT(*) FROM @InputTable);
  DECLARE @CaseStatement VARCHAR(255);

  -- Set the CASE statement to select the appropriate output table based on the value of the `Category` column
  SET @CaseStatement = 'CASE WHEN @Row LIKE = ''aa%'' THEN @OutputTable1 
                             WHEN @Row LIKE = ''bb%'' THEN @OutputTable2
                             WHEN @Row LIKE = ''cc%'' THEN @OutputTable3
                             ELSE @OutputTable4
                        END';

  -- Iterate through the rows in the input table
  WHILE @Count <= @MaxCount
    BEGIN
      -- Get the next row from the input table
      SET @Row = (SELECT TOP (1) * FROM @InputTable ORDER BY [Id] ASC);

      -- Insert the row into the appropriate output table based on the value of the `Category` column
      INSERT INTO 
        (@CaseStatement ([Id], [Category], [Name])) 
      VALUES (@Row.[Id], @Row.[Category], @Row.[Name])

      SET @Count = @Count + 1;
    END
END
GO

However, this approach has a limitation: the table names and column definitions are hardcoded in the stored procedure. If we need to add or remove tables or columns, we would need to modify the stored procedure.

Approach 2: Using Dynamic SQL with Table Names

As an alternative approach, we can use dynamic SQL to generate the queries for inserting each row into its corresponding table.

Here’s an example of how this approach could be implemented:

CREATE FUNCTION dbo.fn_buildDynamicSQL 
(
    @tableName NVARCHAR(50),
    @row NVARCHAR(max)
)
RETURNS NVARCHAR(max)
AS
BEGIN
    
    RETURN 'SELECT * FROM ' + @tableName

END
GO

We can then use this function to generate the dynamic SQL queries for inserting each row into its corresponding table:

DECLARE @t TABLE (col VARCHAR(50))
INSERT INTO @t VALUES ('aa |1|2'), ('bb | 3 | 4 | 5'), ('cc | 6'), ('dd | 7 | 8 | 9 | 0')

SELECT dbo.fn_buildDynamicSQL(tablename, col) AS query
FROM 
    (SELECT CASE LEFT(RTRIM(LTRIM(col)), 2) 
        WHEN 'aa' THEN 'aTable'
        WHEN 'bb' THEN 'bTable'
        WHEN 'cc' THEN 'cTable'
        ELSE 'dTable' END AS tablename, col 
    FROM @t ) c

This approach allows us to dynamically generate the queries for inserting each row into its corresponding table without having to modify the stored procedure.

Conclusion

In this article, we explored two approaches to solving the problem of inserting rows from one table into different tables with varying column definitions. We used dynamic SQL in both approaches to generate the queries for inserting each row into its corresponding table.

The first approach uses a stored procedure with CASE statements to select the appropriate output table based on the value of the Category column. However, this approach has limitations due to the hardcoded table names and column definitions.

The second approach uses a function to dynamically generate the dynamic SQL queries for inserting each row into its corresponding table. This approach provides more flexibility and allows us to easily add or remove tables or columns without modifying the stored procedure.

Ultimately, the choice of approach depends on the specific requirements and constraints of your project.

Last modified on 2024-04-01