Introduction to Oracle Multiset Union Distinct

Oracle Multiset Union Distinct is a powerful tool for combining and de-duplicating data from multiple tables. In this guide, we will provide you with an overview of Oracle Multiset Union Distinct, including its syntax, examples, and best practices.

Syntax

The syntax for Oracle Multiset Union Distinct is as follows:

SELECT column1, column2, …
FROM table1
UNION [DISTINCT | ALL]
SELECT column1, column2, …
FROM table2

Examples

Here are some examples of how to use Oracle Multiset Union Distinct:

Example 1: Basic Usage

Suppose we have two tables, table1 and table2, with the following data:

Table1

| Column1 | Column2 | | ------- | ------- | | A | 1 | | B | 2 | | C | 3 |

Table2

| Column1 | Column2 | | ------- | ------- | | A | 1 | | D | 4 |

We can use the following query to combine the data from both tables and remove duplicates:

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

This will return the following result:

| Column1 | Column2 | | ------- | ------- | | A | 1 | | B | 2 | | C | 3 | | D | 4 |

Example 2: Using the ALL Keyword

If we want to include duplicates in the results, we can use the ALL keyword instead of DISTINCT. For example:

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

This will return the following result:

| Column1 | Column2 | | ------- | ------- | | A | 1 | | B | 2 | | C | 3 | | A | 1 | | D | 4 |

Example 3: Combining Three Tables

We can also use Oracle Multiset Union Distinct to combine data from three or more tables. For example:

Suppose we have three tables, table1, table2, and table3, with the following data:

Table1

| Column1 | Column2 | | ------- | ------- | | A | 1 | | B | 2 | | C | 3 |

Table2

| Column1 | Column2 | | ------- | ------- | | A | 1 | | D | 4 |

Table3

| Column1 | Column2 | | ------- | ------- | | E | 5 | | F | 6 |

We can use the following query to combine the data and remove duplicates:

SELECT column1, column2
FROM table1
UNION DISTINCT
SELECT column1, column2
FROM table2
UNION DISTINCT
SELECT column1, column2
FROM table3;

This will return the following result:

| Column1 | Column2 | | ------- | ------- | | A | 1 | | B | 2 | | C | 3 | | D | 4 | | E | 5 | | F | 6 |

Best Practices

When using Oracle Multiset Union Distinct, keep the following best practices in mind:

• Use DISTINCT if you want to remove duplicates from the result set.
• Use ALL if you want to include duplicates in the result set.
• Make sure the columns in the SELECT statements of each table match in data type and order.
• Use aliases to make column names clear and consistent across tables.
• Test your queries with small data sets before running them on larger data sets to check for any unexpected results.