What is the difference between UNION and UNION ALL?

What is the difference between UNION and UNION ALL?

The UNION and UNION ALL operators in SQL are used to combine the result sets of two or more SELECT statements into a single result set. However, there are key differences between them:

1. Duplicate Rows:

   • UNION: This operator removes duplicate rows from the final result set. When using UNION, SQL will automatically compare all rows from each SELECT statement to identify and remove any duplicates, resulting in a unique set of rows.
   • UNION ALL: This operator does not remove duplicate rows. It will include all rows from each SELECT statement, even if they are identical. This can result in faster query execution as it does not perform the additional step of checking for and removing duplicates.

2. Performance:

   • UNION: Because UNION involves an additional operation to identify and remove duplicates, it generally takes more time to execute than UNION ALL, especially with large datasets where the number of potential duplicates is significant.
   • UNION ALL: Since UNION ALL simply concatenates the results without checking for duplicates, it is typically faster to execute than UNION.

3. Syntax:

   • Both UNION and UNION ALL are used in the same way in the SQL syntax, appended between SELECT statements:

     <code class="sql">SELECT column1, column2 FROM table1
     UNION
     SELECT column1, column2 FROM table2;
     
     SELECT column1, column2 FROM table1
     UNION ALL
     SELECT column1, column2 FROM table2;</code>

4. Ordering:

   • With both UNION and UNION ALL, you can use the ORDER BY clause, but it must be placed at the end of the last SELECT statement. The ORDER BY clause will apply to the entire result set.

Understanding these differences is crucial for choosing the appropriate operator depending on whether you need to eliminate duplicates and the performance requirements of your SQL queries.

How does the performance of UNION compare to UNION ALL?

The performance of UNION and UNION ALL differs mainly due to how they handle duplicate rows:

• UNION: As mentioned earlier, UNION performs an additional operation to identify and remove duplicate rows from the final result set. This involves sorting and comparing the rows from all SELECT statements involved, which can be resource-intensive, especially with large datasets. Consequently, UNION operations are typically slower and more resource-demanding than UNION ALL.
• UNION ALL: UNION ALL, on the other hand, does not perform the additional step of checking for duplicates. It simply concatenates the result sets from each SELECT statement, making it generally faster and more efficient. This speed advantage is particularly noticeable with large datasets where the time spent on duplicate removal could be significant.

In summary, if your query does not require the removal of duplicate rows, using UNION ALL can offer better performance. However, if you need to eliminate duplicates, the slower performance of UNION is the trade-off for ensuring uniqueness in your result set.

Can UNION and UNION ALL be used to combine rows from multiple tables?

Yes, both UNION and UNION ALL can be used to combine rows from multiple tables, provided that the tables have compatible column structures. Here’s how you can use them to combine rows:

1. UNION: You can use UNION to combine rows from multiple tables and eliminate any duplicate rows in the final result set. For example:

   <code class="sql">SELECT column1, column2 FROM table1
   UNION
   SELECT column1, column2 FROM table2
   UNION
   SELECT column1, column2 FROM table3;</code>

   In this example, the columns selected from table1, table2, and table3 must be compatible in terms of data type and number. UNION will then merge these rows into a single result set without any duplicates.

2. UNION ALL: Similarly, UNION ALL can be used to combine rows from multiple tables but will include all rows, including any duplicates. For example:

   <code class="sql">SELECT column1, column2 FROM table1
   UNION ALL
   SELECT column1, column2 FROM table2
   UNION ALL
   SELECT column1, column2 FROM table3;</code>

   Here, the columns selected from table1, table2, and table3 should also be compatible. UNION ALL will concatenate all rows from these tables into a single result set, preserving any duplicate rows.

When using either operator to combine rows from multiple tables, ensure that:

• The number of columns in each SELECT statement is the same.
• The data types of corresponding columns in each SELECT statement are compatible (e.g., you can't combine an integer column with a varchar column).

In conclusion, UNION and UNION ALL are powerful tools for combining data from multiple tables in SQL, offering flexibility in how you manage duplicate rows in the combined result set.

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