EXPLAIN EXTENDED

From Stack Overflow:

I have some data:

id	ref
1	3536757616
1	3536757617
1	3536757618
2	3536757628
2	3536757629
2	3536757630

and want to get the result like this:

id	result
1	3536757616/7/8
2	3536757629/28/30

Essentially, the data should be aggregated on id, and the ref's should be concatenated together and separated by a / (slash), but with longest common prefix removed.

I've already encoutered this problem several times, so I'll try to cover solutions for all RDBMS's my blog is about:

• Longest common prefix: SQL Server
• Longest common prefix: PostgreSQL
• Longest common prefix: Oracle
• Longest common prefix: MySQL

I hope this will be interesting as approaches will differ significantly for all four systems.

Today is SQL Server time.

I won't create sample tables here, since I'm demonstrating the principle. Instead, I'll just use dynamically generated data.

What do we need to do here is:

1. Find least common prefix for each group and its length
2. Cut off the prefix of each but the first ref, using SUBSTRING
3. Concatenate the strings using FOR XML

Steps 2 and 3 are quite simple, but the first one needs some effort.

SQL Server doesn't provide a function to find the longest common prefix (LCP), so we'll have to implement it.

With some limitations, it may be done using pure SQL, no UDF's.
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