How to get the table a FK refers to in Postgres sql - postgresql

Question

For a given table, I am trying to get all the columns, to include their name, type, whether the column is a primary key, or a foreign key, and if it is a FK, what table it points to. I wrote the query below, but it seems to be give me the what the columns are referenced by, instead of the other way around:
select c.column_name, c.udt_name, constraint_type, kcu.table_name as references from information_schema.columns c
left outer join information_schema.constraint_column_usage u on c.column_name=u.column_name
left outer join information_schema.table_constraints t on u.constraint_name=t.constraint_name
left outer join information_schema.key_column_usage AS kcu on t.constraint_name = kcu.constraint_name
where c.table_name=#name
I am not too worried about edge cases, I am just trying to inverse the references column. Thanks for your time.

Answer 1

Here is a query to get all the foreign keys from a given table along with what they point to:
SELECT c.conname,
t1.relname AS from_table,
a1.attname AS from_column,
t2.relname AS to_table,
a2.attname AS to_column
FROM pg_catalog.pg_constraint c,
pg_catalog.pg_class t1,
pg_catalog.pg_class t2,
pg_catalog.pg_attribute a1,
pg_catalog.pg_attribute a2,
pg_catalog.pg_namespace n1,
pg_catalog.pg_namespace n2
WHERE c.conrelid = t1.oid
AND c.confrelid = t2.oid
AND c.contype = 'f'
AND a1.attrelid = t1.oid
AND a1.attnum = ANY(c.conkey)
AND a2.attrelid = t2.oid
AND a2.attnum = ANY(c.confkey)
AND t1.relkind = 'r'
AND t2.relkind = 'r'
AND n1.oid = t1.relnamespace
AND n2.oid = t2.relnamespace
AND n1.nspname NOT IN ('pg_catalog', 'pg_toast')
AND n2.nspname NOT IN ('pg_catalog', 'pg_toast')
AND pg_catalog.pg_table_is_visible(t1.oid)
AND pg_catalog.pg_table_is_visible(t2.oid)
AND t1.relname = #name;
I see you are using information_schema instead of pg_catalog, so the easiest way might be to turn my query into a CTE or subquery, then join to it based on your table and column name.
EDIT: It sounds like you just need some help joining my query to what you already have. Actually your query has a lot of errors in how it does the joins, so I've rewritten it to include the correct foreign key results:
SELECT c.column_name,
c.udt_name,
t.constraint_type,
t.constraint_name,
x.to_table
FROM information_schema.columns c
LEFT OUTER JOIN (
SELECT t.constraint_type,
t.constraint_catalog,
t.constraint_schema,
t.constraint_name,
t.table_catalog,
t.table_schema,
t.table_name,
u.column_name
FROM information_schema.constraint_column_usage u
LEFT OUTER JOIN information_schema.table_constraints t
ON t.table_catalog = u.table_catalog
AND t.table_schema = u.table_schema
AND t.table_name = u.table_name
AND t.constraint_catalog = u.constraint_catalog
AND t.constraint_schema = u.constraint_schema
AND t.constraint_name = u.constraint_name
WHERE t.constraint_type IS DISTINCT FROM 'FOREIGN KEY'
) t
ON c.table_catalog = t.table_catalog
AND c.table_schema = t.table_schema
AND c.table_name = t.table_name
AND c.column_name = t.column_name
LEFT OUTER JOIN (
SELECT c.conname,
t1.relname AS from_table,
a1.attname AS from_column,
t2.relname AS to_table,
a2.attname AS to_column
FROM pg_catalog.pg_constraint c,
pg_catalog.pg_class t1,
pg_catalog.pg_class t2,
pg_catalog.pg_attribute a1,
pg_catalog.pg_attribute a2,
pg_catalog.pg_namespace n1,
pg_catalog.pg_namespace n2
WHERE c.conrelid = t1.oid
AND c.confrelid = t2.oid
AND c.contype = 'f'
AND a1.attrelid = t1.oid
AND a1.attnum = ANY(c.conkey)
AND a2.attrelid = t2.oid
AND a2.attnum = ANY(c.confkey)
AND t1.relkind = 'r'
AND t2.relkind = 'r'
AND n1.oid = t1.relnamespace
AND n2.oid = t2.relnamespace
AND n1.nspname NOT IN ('pg_catalog', 'pg_toast')
AND n2.nspname NOT IN ('pg_catalog', 'pg_toast')
AND pg_catalog.pg_table_is_visible(t1.oid)
AND pg_catalog.pg_table_is_visible(t2.oid)
) x
ON x.from_table = c.table_name
AND x.from_column = c.column_name
WHERE c.table_name = #cards
;
This version also prevents lots of duplicate rows if you have NOT NULL or CHECK constraints.

Answer 2

I am not sure if you know how to programming with other languages(such as java, C#) besides writing Sql queries
If you know any of them, you can use the function which are int the JDBC/ODBC drivers to get the things you want
Here is an example of using jdbc
you open a connection to the database
from the connection you get, and you can get the meta data, here is the code
conn=dataSource.getConnection();
DatabaseMetaData dbmeta = conn.getMetaData();
After you get the meta data, you can get all the columns, primary keys, foreign keys and even indexed. See the sample codes below
ResultSet rsKey = dbmeta.getPrimaryKeys(null, schemaName, tableName);
ResultSet rs = dbmeta.getColumns(null, schemaName, tableName, "%");
ResultSet rsForeign=meta.getExportedKeys(null, schemaName, tableName);