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UNION and UNION ALL

The UNION operator combines the result sets of 2 or more SELECT statements and removes duplicate rows between the various SELECT statements.

The UNION ALL operator combines the result sets of 2 or more SELECT statements and returns all rows from the query. It does not remove duplicate rows between the various SELECT statements.

Each SELECT statement within the UNION operator must have the same number of fields in the result sets with similar data types.

The syntax for the UNION ALL operator is as follows:

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
UNION ALL
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];

expression1, expression2, … expression_n are the columns or calculations you wish to retrieve.

tables are the tables that you wish to retrieve records from. There must be at least one table listed in the FROM clause.

WHERE conditions are optional. These conditions must be met for the records to be selected.

Suppose that we have a table,points_scored_current_week, that consists of these columns: id, first_half, and second_half.

Next, suppose that we have a second table, points_scored_last_week, that consists of these columns: id, first_half, and second_half.

Here is an example that uses the UNION operator:

SELECT *
FROM points_scored_current_week
UNION
SELECT *
FROM points_scored_last_week;

The result looks like this:

|  id   |first_half|second_half|
|-------|----------+-----------+
|   1   |    10    |    20     |

Here is an example that uses the UNION ALL operator:

SELECT *
FROM points_scored_current_week
UNION ALL
SELECT *
FROM points_scored_last_week;

The result looks like this:

|  id   |first_half|second_half|
|-------|----------+-----------+
|   1   |    10    |    20     |
|   2   |    10    |    20     |

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INTERSECT

The INTERSECT operator combines the result sets of 2 or more SELECT statements and returns only the rows that are common to all the SELECT statements. It removes duplicate rows from the final result set.

Each SELECT statement within the INTERSECT operator must have the same number of fields in the result sets with similar data types.

The syntax for the INTERSECT operator is as follows:

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
INTERSECT
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];

expression1, expression2, … expression_n are the columns or calculations you wish to retrieve.

tables are the tables that you wish to retrieve records from. There must be at least one table listed in the FROM clause.

WHERE conditions are optional. These conditions must be met for the records to be selected.

Suppose that we have a table,points_scored_current_week, that consists of these columns: id, first_half, and second_half.

Next, suppose that we have a second table, points_scored_last_week, that consists of these columns: id, first_half, and second_half.

Here is an example that uses the INTERSECT operator:

SELECT *
FROM points_scored_current_week
INTERSECT
SELECT *
FROM points_scored_last_week;

The result looks like this:

|  id   |first_half|second_half|
|-------|----------+-----------+
|   1   |    10    |    20     |

In this case, the INTERSECT operator returned the rows that are common to both the points_scored_current_week and points_scored_last_week tables. If there were no common rows, the INTERSECT operator would return an empty set.

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CORRESPONDING in set operations

Set operations (UNION, INTERSECT, and EXCEPT) typically require:

• Both queries to return the same number of columns
• Columns to match in left-to-right order (by column index)

However, you can use the CORRESPONDING keyword to match columns by name instead of order. This approach:

• Only combines columns that exist in both sets
• Ignores columns not present in both sets
• Considers columns matching if they have the same name or alias

Overall, using CORRESPONDING gives you more flexibility when performing set operations, as it doesn’t rely on strict column ordering.

The syntax for using CORRESPONDING is as below:

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
<operation> CORRESPONDING [BY (column_name1, column_name2, ...)]
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];

<operation> is one of the below operations:

UNION [ALL] | INTERSECT | EXCEPT

If you want to explicitly specify the columns to match, use the CORRESPONDING BY specification. Only columns that are specified and exist on both sides will be overlaid.

Here is a simple example. First, let’s create two tables employees and managers, and insert some data. Then you can use the CORRESPONDING keyword.

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR,
    age INT,
    gender VARCHAR
);

CREATE TABLE managers (
    id INT PRIMARY KEY,
    name VARCHAR,
    salary DECIMAL,
    gender VARCHAR
);

INSERT INTO employees (id, name, age, gender) VALUES
(1, 'Alice', 30, 'Female'),
(2, 'Bob', 25, 'Male'),
(3, 'Charlie', 28, 'Male');

INSERT INTO managers (id, name, salary, gender) VALUES
(1, 'David', 80000, 'Male'),
(2, 'Eve', 90000, 'Female'),
(3, 'Frank', 75000, 'Male');

-- Not specifying the columns to match. Columns id, name, gender will be overlaid.
SELECT id, name, age, gender
FROM employees
UNION CORRESPONDING
SELECT id, name, salary, gender
FROM managers;

----RESULT
id |  name   | gender
----+---------+--------
  1 | Alice   | Female
  3 | Frank   | Male
  1 | David   | Male
  3 | Charlie | Male
  2 | Eve     | Female
  2 | Bob     | Male
(6 rows)

-- Specify the columns to match. Only id and name columns will be overlaid.
SELECT id, name, age, gender
FROM employees
UNION CORRESPONDING BY (id, name)
SELECT id, name, salary, gender
FROM managers;

----RESULT
 id |  name
----+---------
  3 | Charlie
  1 | Alice
  3 | Frank
  1 | David
  2 | Eve
  2 | Bob
(6 rows)