When it comes to querying databases, the SELECT statement is one of the most commonly used commands. It allows users to retrieve data from a database table or view based on specific criteria. But what about using the DISTINCT keyword on multiple columns? Is it possible to use it with more than one column in a single query? Let's find out.
First, let's understand what the DISTINCT keyword does. It is used to eliminate duplicate records from the result set. For example, if we have a table with the following data:
| ID | Name | Age |
|----|------|-----|
| 1 | John | 25 |
| 2 | John | 25 |
| 3 | Mary | 30 |
If we use the SELECT DISTINCT statement on the Name column, we will get the following result:
| Name |
|------|
| John |
| Mary |
As you can see, the duplicate record for John has been eliminated. Now, let's see if we can use this keyword on multiple columns.
The short answer is yes, we can use SELECT DISTINCT on multiple columns. However, the results might not be what we expect. Let's use the same example as before, but this time let's use the DISTINCT keyword on both the Name and Age columns.
| ID | Name | Age |
|----|------|-----|
| 1 | John | 25 |
| 2 | John | 25 |
| 3 | Mary | 30 |
SELECT DISTINCT Name, Age FROM table_name;
The result we get is:
| Name | Age |
|------|-----|
| John | 25 |
| John | 25 |
| Mary | 30 |
As you can see, the DISTINCT keyword has eliminated the duplicate records based on both the Name and Age columns. However, it has not combined the values of the two columns. This means that we still have two rows for John with the same age of 25. This is because the DISTINCT keyword looks at each column individually and not as a combination.
But what if we want to get a unique combination of the Name and Age columns? For this, we can use the GROUP BY clause. It is used to group the result set by one or more columns. Let's see how it works in our example.
SELECT Name, Age FROM table_name GROUP BY Name, Age;
The result we get is:
| Name | Age |
|------|-----|
| John | 25 |
| Mary | 30 |
As you can see, we now have a unique combination of the Name and Age columns. This is because the GROUP BY clause combines the values of both columns and eliminates the duplicates. It is important to note that the order of the columns in the GROUP BY clause matters. If we were to switch the order, we would get a different result.
In conclusion, yes, it is possible to use SELECT DISTINCT on multiple columns, but the results might not always be what we expect. Using the GROUP BY clause can help us get a unique combination of the columns. It is always important to carefully consider the use of these keywords and clauses to get the desired result from our database queries.
