Here are the queries to find primary key of a table in Postgres, SQL Server and Oracle. Replace the <TABLE_NAME> with the table name that we are interested in.
Postgre
SELECT a.attname, format_type(a.atttypid, a.atttypmod) AS data_type
FROM pg_index i
JOIN pg_attribute a ON a.attrelid = i.indrelid
AND a.attnum = ANY(i.indkey)
WHERE i.indrelid = '<TABLE_NAME>'::regclass
AND i.indisprimary;
SQL Server
select C.COLUMN_NAME FROM
INFORMATION_SCHEMA.TABLE_CONSTRAINTS T
JOIN INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE C
ON C.CONSTRAINT_NAME=T.CONSTRAINT_NAME
WHERE
C.TABLE_NAME='<TABLE_NAME>'
and T.CONSTRAINT_TYPE='PRIMARY KEY'
Oracle
SELECT COLUMN_NAME FROM all_cons_columns WHERE constraint_name = (
SELECT constraint_name FROM user_constraints
WHERE UPPER(table_name) = UPPER('<TABLE_NAME>') AND CONSTRAINT_TYPE = 'P')
