SQL Merge Structure for Handling Non-Matching Sources in Filtered Parent-Child Tables

Data and Databases
1

I have two tables representing a PARENT-CHILD hierarchy, as well as their respective ‘snapshot’ versions. The table structures are as follows:

CREATE TABLE Parent(
  ParentId INT
);

CREATE TABLE Child(
  ChildId INT,
  ParentId INT,
  ColA INT
);

CREATE TABLE ParentSnapshot(
  ParentSnapshotId INT,
  ParentId INT
);

CREATE TABLE ChildSnapshot(
  ChildSnapshotId INT,
  ParentSnapshotId INT,
  ChildId INT,
  ParentId INT,
  ColA INT
);

At a certain point, records from the Parent and Child tables are duplicated into ParentSnapshot and ChildSnapshot. Now, I want to sync ChildSnapshot with the corresponding data from the Child table based on a specific list of ParentSnapshotIds.

What steps should I follow to write a merge statement that:

  1. Inserts new records into ChildSnapshot if they were added to the Child table,
  2. Deletes records from ChildSnapshot that no longer exist in the Child table, and
  3. Updates existing records that match?

I am having difficulty formulating the correct WHERE clause for filtering by ParentSnapshotIds and the corresponding ParentIds.

2

hm, this is quite an interesting challenge! are there specific conditions or transformations for the “ColA” column when syncing between Child and ChildSnapshot? i’m curious about how you manage dependencies between snapshots and their original records. have you considered using CTEs or temp tables for intermediary steps?

3

When working on a MERGE operation like this, ensure you have a clear understanding of the associations and filtering criteria. Firstly, you need to map the ParentSnapshotId to ParentId using a subquery or join within the MERGE statement’s USING clause. This ensures records are correctly aligned between Parent and Child tables. You can utilize a correlated subquery in your ON clause to pull ParentSnapshotIds based on ParentIds from the ParentSnapshot table. Also, remember to detail the INSERT, UPDATE, and DELETE logic based on whether the target record exists or not. Make sure any updates or deletes are specifically synchronized to keys obtained through your mapping logic and contain only the necessary conditions for ParentSnapshotIds.

4

well, one think to consider is checking for any duplication issues that might arise. if you’ve got constraints in place over keys or unique cols when merging, it could cause problems during the transactional update. also, look at using EXECUTE AS or permissions if access to specific sql ops is blocked. good luck!

5

Have you thoght about the perfomance when doing this merge on larger datasets? I’m curious how indexes and partitions might affect the speed and efficiency of your operations. Do all your tables have appropriate INDEXES set up for the merge task? It’d be interesting to discuss that angle too!