Sql-server – Splitting data into two tables, in one go

sql serversql-server-2008t-sql

I am using SQL Server 2008 and have a problem which I don't know how to solve without using many temporary tables and an unreliable join.

Table 1 contains 6 columns of data, which is then split into two tables. Col1 to Col3 goes into Table 2, and Col4 to Col6 goes into Table 3. Getting the data into Table 2 and Table 3 is the easy part. However T2ID in Table 3 is a foreign key to the ID in Table 2.

Performance is key so I don't want to use variables and/or iterate through the data row by row, ideally I just want one insert which does the lot.

I've tried using a Link Table but the data in Table 2 and Table 3 is not unique which makes joining unreliable.

Any suggestions?

Create Table T1 (
  ID INT IDENTITY(1,1),
  Col1 VARCHAR(10),
  Col2 VARCHAR(10),
  Col3 VARCHAR(10),
  Col4 VARCHAR(10),
  Col5 VARCHAR(10),
  Col6 VARCHAR(10)
)

Create Table T2 (
  ID INT IDENTITY(1,1),
  Col1 VARCHAR(10),
  Col2 VARCHAR(10),
  Col3 VARCHAR(10)
)

Create Table T3 (
  ID INT IDENTITY(1,1),
  T2ID INT,
  Col4 VARCHAR(10),
  Col5 VARCHAR(10),
  Col6 VARCHAR(10)
)

I can't change T2 or T3. T1 is not a staging Table, and I can't alter that either. There are other packages also writing to T1, T2, and T3. Although it is possible to schedule them at different times should I need to.

Best Answer

If you use MERGE to insert the data into T2, you can generate a mapping table between T1.ID and T2.ID:

DECLARE @Mapping TABLE
(
  T1ID int,
  T2ID int
);
MERGE INTO
  dbo.T2 AS tgt
USING
  dbo.T1 AS src
ON
  1 = 0
WHEN NOT MATCHED THEN
  INSERT (    Col1,     Col2,     Col3)
  VALUES (src.Col1, src.Col2, src.Col3)
OUTPUT
  src.ID, inserted.ID INTO @Mapping (T1ID, T2ID)
;

Unlike INSERT, which would let you reference the inserted table's columns only in the OUTPUT clause, the MERGE statement allows you to reference the source table's columns as well. That is key to this solution, because that is how you associate the source IDs with the target IDs.

Once you have the mapping table, you can just use it in a join when inserting into T3. A plain INSERT ... SELECT will do this time:

INSERT INTO
  dbo.T3 (T2ID, Col4, Col5, Col6)
SELECT
  m.T2ID,
  t.Col4,
  t.Col5,
  t.Col6
FROM
  dbo.T1 AS t
  INNER JOIN @Mapping AS m ON t.ID = m.T1ID
;

Wrap both statements in one transaction to make your split operation atomic.

Discussion about the MERGE mapping method can be found in this thread:

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