How to Union All In Postgresql?

In PostgreSQL, you can use the UNION ALL operator to combine the results of two or more SELECT statements into a single result set. This allows you to merge rows from multiple tables or queries that have the same columns.

The syntax for using UNION ALL in PostgreSQL is as follows: SELECT column1, column2, ... FROM table1 UNION ALL SELECT column1, column2, ... FROM table2;

In this example, table1 and table2 are the names of the tables you want to combine the results from, and column1, column2, etc. are the columns you want to display in the final result set.

It is important to note that when using UNION ALL, all SELECT statements must have the same number of columns, and the columns must have compatible data types. Additionally, UNION ALL will include duplicate rows in the final result set, whereas the UNION operator will remove duplicates.

What is the difference between UNION and UNION ALL in PostgreSQL?

In PostgreSQL, UNION and UNION ALL are both used to combine the result sets of two or more SELECT statements. However, there is one key difference between them:

UNION: This operator combines the result sets of the SELECT statements and removes any duplicate rows from the result set. This means that if two or more SELECT statements retrieve the same row of data, it will only appear once in the final result set.

UNION ALL: This operator also combines the result sets of the SELECT statements, but it does not remove duplicate rows from the result set. This means that if two or more SELECT statements retrieve the same row of data, it will appear multiple times in the final result set.

In summary, UNION removes duplicate rows from the result set, while UNION ALL includes all rows from the result sets of the SELECT statements.

What is the syntax for UNION ALL in PostgreSQL?

The syntax for UNION ALL in PostgreSQL is as follows:

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SELECT column1, column2, ...
FROM table1
UNION ALL
SELECT column1, column2, ...
FROM table2;

This will combine the results of the two SELECT queries and include all rows from both tables.

How to use UNION ALL with CASE statements in PostgreSQL?

To use UNION ALL with CASE statements in PostgreSQL, you can follow these steps:

1. Write your SQL query with the CASE statement inside the SELECT clause. For example, consider the following query:

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SELECT
    id,
    name,
    CASE
        WHEN age < 18 THEN 'Minor'
        WHEN age < 65 THEN 'Adult'
        ELSE 'Senior'
    END AS age_group
FROM
    users

1. Use the above query as a subquery and combine it using UNION ALL with another query that also has a CASE statement. For example:

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SELECT * FROM (
    SELECT
        id,
        name,
        CASE
            WHEN age < 18 THEN 'Minor'
            WHEN age < 65 THEN 'Adult'
            ELSE 'Senior'
        END AS age_group
    FROM
        users

    UNION ALL

    SELECT
        id,
        name,
        CASE
            WHEN gender = 'M' THEN 'Male'
            WHEN gender = 'F' THEN 'Female'
            ELSE 'Other'
        END AS gender
    FROM
        users
) AS combined_results

1. In the above example, we have combined two queries using UNION ALL. The first query categorizes users into different age groups using the CASE statement, while the second query categorizes users based on their gender using another CASE statement.
2. Execute the combined query, and you will get the results from both queries combined using UNION ALL.

By following these steps, you can use UNION ALL with CASE statements in PostgreSQL to combine the results of multiple queries with different CASE logic.

How to use UNION ALL with aggregate functions in PostgreSQL?

To use UNION ALL with aggregate functions in PostgreSQL, you can combine the results of multiple SELECT statements using the UNION ALL operator and then apply aggregate functions to the combined result set. Here's an example:

1. Create two tables with sample data:

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CREATE TABLE table1 (
    id SERIAL PRIMARY KEY,
    value INT
);

CREATE TABLE table2 (
    id SERIAL PRIMARY KEY,
    value INT
);

INSERT INTO table1(value) VALUES (1), (2), (3);
INSERT INTO table2(value) VALUES (4), (5), (6);

1. Execute a query using UNION ALL and an aggregate function:

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SELECT SUM(value) as total_value
FROM (
    SELECT value FROM table1
    UNION ALL
    SELECT value FROM table2
) combined_values;

In this query, we first combine the values from table1 and table2 using UNION ALL, creating a subquery called combined_values. Then, we apply the SUM aggregate function to calculate the total value of all the combined rows.

This will return the total sum of values from both tables:

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total_value
-------------
21

You can also use other aggregate functions like COUNT, AVG, MIN, MAX, etc. in a similar way with UNION ALL to perform aggregate operations on combined result sets from multiple tables.