How to Compare two Tables in PostgreSQL

Summary: in this tutorial, you will learn various ways to compare two tables in PostgreSQL.

There are several ways to compare the content of two tables to find the differences between them. We will show you two commonly used techniques to compare the data from two tables.

Comparing two tables using EXCEPT and UNION operators

First, create table two tables called foo and bar, and insert some sample data for demonstration purposes:

CREATE TABLE foo (
  id INT PRIMARY KEY,
  name VARCHAR (50)
);

CREATE TABLE bar (
  id INT PRIMARY KEY,
  name VARCHAR (50)
);

INSERT INTO foo (id, name)
VALUES
  (1, 'a'),
  (2, 'b');

INSERT INTO bar (id, name)
VALUES
  (1, 'a'),
  (2, 'b');

The foo table has the same structure and data as the bar table.

Next, update one row in the bar table.

UPDATE
  bar
SET
  name = 'c'
WHERE
  id = 2;

compare-two-tables-postgresql Then, find the rows in the foo table but not in the bar table using the following query:

SELECT
  id,
  name,
  'not in bar' AS note
FROM
  foo
EXCEPT
SELECT
  id,
  name,
  'not in bar' AS note
FROM
  bar;

Output:

id | name |    note
----+------+------------
  2 | b    | not in bar
(1 row)

We used the EXCEPT operator that returns the rows in the foo table but not in the bar table. We can apply the same technique to find the rows that are in the bar table but not in the foo table.

SELECT
  ID,
  NAME,
  'not in foo' AS note
FROM
  bar
EXCEPT
SELECT
  ID,
  NAME,
  'not in foo' AS note
FROM
  foo;

Output:

id | name |    note
----+------+------------
  2 | c    | not in foo
(1 row)

Finally, use the UNION operator to combine the results of both queries to find the rows in the bar table but not in the foo table and vice versa:

SELECT
  id,
  name,
  'not in bar' AS note
FROM
  foo
EXCEPT
SELECT
  id,
  name,
  'not in bar' AS note
FROM
  bar
UNION
SELECT
  ID,
  NAME,
  'not in foo' AS note
FROM
  bar
EXCEPT
SELECT
  ID,
  NAME,
  'not in foo' AS note
FROM
  foo;

Output:

id | name |    note
----+------+------------
  2 | c    | not in foo
  2 | b    | not in bar
(2 rows)

Comparing two tables using an outer join

You can use the outer join to compare two tables as follows:

SELECT
  id,
  name
FROM
  foo FULL
  OUTER JOIN bar USING (id, name)
WHERE
  foo.id IS NULL
  OR bar.id IS NULL;

It returns the differences between the two tables:

id | name
----+------
  2 | b
  2 | c
(2 rows)

To find the number of rows that are in the foo table but not bar table and vice versa, you use the COUNT function as follows:

SELECT
  COUNT (*)
FROM
  foo FULL
  OUTER JOIN bar USING (id, name)
WHERE
  foo.id IS NULL
  OR bar.id IS NULL;

Output:

count
-------
     2
(1 row)

Summary

Use UNION and EXCEPT operators or outer join to compare two tables in PostgreSQL.

How to Compare Two Tables in PostgreSQL

Comparing two tables using EXCEPT and UNION operators

Comparing two tables using an outer join

Summary