Question

This might sound a bit un-realistic, but I’ve recently had to study the database structure of a system am maintaining remotely, and it hit me that there were some occassions when I wanted to find all columns that were linked to a given table (see, the original devs didn’t create explicit relations, but instead have relations encoded in the column names!).

For example, assuming all columns referring to a column parent.parent have the form table.somefield_parent as the means of documenting a relationship between the two fields, how do I find all such relationships (and the tables involved) using say the standard SQL commands / in-built functions?

Any solutions for MySQL or PostgreSQL are welcome.

NOTE :

• am not looking for patterns in the data, but in the meta-data.
• I know I can dump the db structure and search for the results there, but I don’t want to go out of the db-shell to find these relationships (assume am working in say psql, and don’t want to exit)

Answer 1

In PostgreSQL you can query the system catalog tables.

You can also query the information_schema, which consists of views providing the standard SQL way of displaying the information. Querying the system catalogs is usually much faster, though.

To find the children of one particular parent column:

SELECT n.nspname AS schema_name
     , c.relname AS table_name
     , a.attname AS column_name
FROM   pg_catalog.pg_attribute a
JOIN   pg_catalog.pg_class c ON c.oid = a.attrelid
JOIN   pg_catalog.pg_namespace n ON n.oid = c.relnamespace
WHERE  a.attname ~~ E'%\\_parent'
AND    NOT a.attisdropped
AND    c.relkind = 'r'
AND    nspname !~~ E'pg\\_%';

To find all related column according to the description:

SELECT n.nspname AS parrent_schema
     , c.relname AS parrent_table
     , a.attname AS parrent_column
     , n1.nspname AS child_schema
     , c1.relname AS child_table
     , a1.attname AS child_column
FROM   pg_catalog.pg_attribute a
JOIN   pg_catalog.pg_class c ON c.oid = a.attrelid
JOIN   pg_catalog.pg_namespace n ON n.oid = c.relnamespace
JOIN   pg_catalog.pg_attribute a1 ON a.attname = substring(a1.attname, '_(.*?)$')
JOIN   pg_catalog.pg_class c1 ON c1.oid = a1.attrelid
JOIN   pg_catalog.pg_namespace n1 ON n1.oid = c1.relnamespace
WHERE  c.relkind = 'r'
AND    c1.relkind = 'r'
AND    n.nspname !~~ E'pg\\_%'
AND    n1.nspname !~~ E'pg\\_%'
AND    NOT a.attisdropped
AND    NOT a1.attisdropped
-- AND    n.nspname = 'public'   -- to narrow it down to a specific schema
-- AND    n1.nspname = 'public'   -- to narrow it down to a specific schema
ORDER  BY 1,2,3,4,5,6

This assumes that the parrent columns don’t have _ in the name.
The key element is the join condition:

a.attname = substring(a1.attname, '_(.*?)$')