Difference Between Union And Union All

When working with relational databases, it's important to understand the different ways in which tables can be combined. Two commonly used methods for combining tables are UNION and UNION ALL. Although they may seem similar, there are important differences between the two that can affect the results of your queries.

UNION

UNION is a method of combining the rows of two or more tables into a single result set. When using UNION, only distinct values are returned. In other words, if there are duplicate values in the tables being combined, they will only appear once in the final result set.

Here is an example query:

```

SELECT column_name(s) FROM table1

UNION

SELECT column_name(s) FROM table2;

```

This query will return a result set that combines the rows from table1 and table2, but will only include distinct values. If there are any duplicate values between the two tables, only one instance of each value will be included in the result set.

It's important to note that when using UNION, the number and order of the columns in the SELECT statements must be the same for each table being combined. If they are different, you will receive an error.

UNION ALL

UNION ALL is another method of combining tables. However, unlike UNION, it does not remove duplicates. Instead, it simply combines all rows from the tables being joined and returns them as one result set.

Here is an example query:

```

SELECT column_name(s) FROM table1

UNION ALL

SELECT column_name(s) FROM table2;

```

This query will return a result set that combines all rows from table1 and table2, including duplicates. This means that if there are any duplicate values between the two tables, they will both be included in the final result set.

One important thing to note is that UNION ALL is generally faster than UNION, as it does not need to check for duplicates. However, this can also lead to larger result sets, as duplicates are not removed.

Which one to use?

So, which method should you use? It really depends on the specific needs of your query. If you want to combine tables and remove any duplicate values, use UNION. If you want to combine tables and include all rows, regardless of duplicates, use UNION ALL.

It's also important to consider the impact of duplicates on your query results. In some cases, duplicates may be important and you'll want to include them in your result set. In other cases, duplicates may skew your results, and you'll want to remove them.

Here's an example scenario to help illustrate the difference:

Imagine you have two tables - one table contains a list of customers, and the other contains a list of orders. You want to combine these two tables in order to see a list of all customers who have placed orders.

If you use UNION, you'll get a result set that includes each customer only once, even if they've placed multiple orders. This may be useful if you're trying to get a list of unique customers.

If you use UNION ALL, you'll get a result set that includes each customer for each order they've placed. This may be useful if you're trying to get a complete list of all orders, and need to know which customers placed them.

In general, you'll want to use UNION when you're trying to get a distinct set of values, and UNION ALL when you're trying to get a complete set of values.

Summary

In summary, UNION and UNION ALL are two methods of combining tables in relational databases. UNION removes duplicates from the result set, while UNION ALL includes all rows, regardless of duplicates. It's important to use the method that meets the specific needs of your query. Understanding the differences between these two methods can help you write more efficient and accurate SQL queries.