How to Get Distinct Record Postgresql With Union?

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To get distinct records in PostgreSQL using UNION, you can use the keyword DISTINCT after SELECT in each query that you are combining with UNION. This will ensure that only unique records are returned from the combined result set. The DISTINCT keyword will remove any duplicate records that are returned from the queries being combined with UNION.


What is the result of UNION operation in PostgreSQL when using the DISTINCT keyword?

When using the DISTINCT keyword with the UNION operation in PostgreSQL, the result will remove any duplicate rows from the combined result set of the UNION operation. This means that the final result will only contain unique rows from both sets.


What is the syntax for using the UNION operator in PostgreSQL?

The syntax for using the UNION operator in PostgreSQL is as follows:


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


This query will combine the results of the two SELECT statements and remove any duplicate rows.


How to combine results from multiple SELECT statements in PostgreSQL?

There are multiple ways to combine results from multiple SELECT statements in PostgreSQL:

  1. UNION: The UNION operator is used to combine the results of two or more SELECT statements into a single result set. The columns in each SELECT statement must have the same data types and be in the same order.
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SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2;


  1. UNION ALL: The UNION ALL operator is similar to UNION, but it includes duplicate rows in the result set.
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SELECT column1, column2 FROM table1
UNION ALL
SELECT column1, column2 FROM table2;


  1. JOIN: You can also use JOIN to combine results from multiple SELECT statements if the tables have a common column that can be used to join them.
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SELECT t1.column1, t1.column2, t2.column3 FROM table1 t1
JOIN table2 t2 ON t1.common_column = t2.common_column;


  1. Cross Join: If you want to combine results without any specific join condition, you can use a cross join.
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SELECT table1.column1, table2.column2 FROM table1
CROSS JOIN table2;


  1. Using CTEs (Common Table Expressions): You can use Common Table Expressions to define temporary result sets that can be used in subsequent SELECT statements.
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WITH cte1 AS (
    SELECT column1, column2 FROM table1
),
cte2 AS (
    SELECT column1, column2 FROM table2
)
SELECT * FROM cte1
UNION
SELECT * FROM cte2;


These are some of the ways you can combine results from multiple SELECT statements in PostgreSQL. Choose the method that best suits your requirements and data structure.


How to use the LIMIT clause with UNION and DISTINCT in PostgreSQL?

To use the LIMIT clause with UNION and DISTINCT in PostgreSQL, you can follow these steps:

  1. Write the individual queries that you want to combine using UNION. Make sure to include the DISTINCT keyword in each query to remove duplicate rows.
  2. Use the UNION keyword to combine the queries. Place the individual queries inside parentheses and separate them with UNION.
  3. Add the LIMIT clause at the end of the combined query to limit the number of rows returned.


Here is an example query that demonstrates how to use the LIMIT clause with UNION and DISTINCT in PostgreSQL:

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(SELECT column_name FROM table1 WHERE condition1
   UNION
 SELECT column_name FROM table2 WHERE condition2)
   UNION
 SELECT column_name FROM table3 WHERE condition3
   UNION
 SELECT column_name FROM table4 WHERE condition4
   ORDER BY column_name
   LIMIT 10;


In this example, we have combined four individual queries using UNION and applied the LIMIT clause to limit the result set to 10 rows. The DISTINCT keyword ensures that any duplicate rows are removed before applying the LIMIT clause.


What is the role of indexes in improving query performance with UNION and DISTINCT in PostgreSQL?

Indexes in PostgreSQL play a crucial role in improving query performance by helping the database quickly locate and retrieve the data required by queries. When using UNION and DISTINCT in queries, indexes can help to optimize the performance by reducing the number of rows that need to be processed.


When UNION and DISTINCT are used in a query, PostgreSQL needs to combine and de-duplicate the results from multiple tables or queries. Indexes can be utilized to speed up this process by allowing the database to quickly identify and eliminate duplicate rows or merge the results from different tables.


By creating indexes on the columns involved in the UNION and DISTINCT operations, PostgreSQL can efficiently handle the sorting and de-duplicating of data, resulting in faster query performance. Indexes can also help to reduce the amount of data that needs to be scanned and processed, leading to quicker query execution times.


Overall, indexes in PostgreSQL can significantly improve query performance when using UNION and DISTINCT by enabling the database to efficiently process and retrieve the desired data, ultimately leading to faster and more efficient query execution.

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