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 data of two tables.
Compare two tables using EXCEPT and UNION operators
First, let’s create table two tables named foo and bar, and insert some sample data for the demonstration.
1 2 3 4 | CREATE TABLE foo ( ID INT PRIMARY KEY, NAME VARCHAR (50) ); |
1 2 3 4 | INSERT INTO foo (ID, NAME) VALUES (1, 'a'), (2, 'b'); |
1 2 3 4 | CREATE TABLE bar ( ID INT PRIMARY KEY, NAME VARCHAR (50) ); |
1 2 3 4 | INSERT INTO bar (ID, NAME) VALUES (1, 'a'), (2, 'b'); |
The foo table has the same structure and data as the bar table.
Next, we update one row in the bar table.
1 2 3 4 | UPDATE bar SET name = 'c' WHERE id = 2; |
Then, to find the rows in the foo table but not in the bar table, we use the following query:
1 2 3 4 5 6 7 8 9 10 11 12 13 | SELECT ID, NAME, 'not in bar' AS note FROM foo EXCEPT SELECT ID, NAME, 'not in bar' AS note FROM bar |
We used 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.
1 2 3 4 5 6 7 8 9 10 11 12 13 | SELECT ID, NAME, 'not in foo' AS note FROM bar EXCEPT SELECT ID, NAME, 'not in foo' AS note FROM foo |
Finally, we use UNION operator to combine the result of both queries to find:
- Rows in the
bartable but not in thefootable - Rows in the
footable but not in thebartable.
Compare two tables using OUTER JOIN
We can use the outer join to compare two tables as follows:
1 2 3 4 5 6 7 8 9 | 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 two tables:
To find the number of rows that are in the foo table but not bar table and vice versa, we use the COUNT function as follows:
1 2 3 4 5 6 7 8 | SELECT COUNT (*) FROM foo FULL OUTER JOIN bar USING (id, name) WHERE foo.id IS NULL OR bar.id IS NULL; |
In this tutorial, we have shown you two ways to compare two tables in PostgreSQL.