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DISTINCT SQL operator
The DISTINCT SQL operator is used to remove duplicates from the resulting SELECT operator set.
Syntax for DISTINCT statement in SQL
SELECT DISTINCT exprs
FROM tabs
[WHERE conds];
where:
- exprs – Columns or calculations that you want to get.
- tabs – The tables from which you want the records. The FROM sentence must contain at least one table.
- WHERE conditions – It’s optional. Conditions to be met for the records to be selected.
If only one expression is specified in the DISTINCT operator, the query will return unique values for that expression.
If multiple expressions are specified in the DISTINCT operator, the query retrieves the unique combinations for the listed expressions.
In SQL, the DISTINCT operator does not ignore NULL values. So when using DISTINCT in your SQL statement, your resulting set will contain the value of NULL as a separate value.
DISTINCT – search for unique values in a column
Let’s see how to use the DISTINCT operator to find unique values in a single table column.
In this example, we have a table with the following data:
| suppl_id | suppl_name | city_id | state_id |
|---|---|---|---|
| 100 | Yandex | Moscow | Russia |
| 200 | Lansing | Michigan | |
| 300 | Oracle | Redwood City | California |
| 400 | Bing | Redmond | Washington |
| 500 | Yahoo | Sunnyvale | Washington |
| 600 | DuckDuckGo | Paoli | Pennsylvania |
| 700 | Qwant | Paris | Ile de France |
| 800 | Menlo Park | California | |
| 900 | Electronic Arts | San Francisco | California |
Let’s find all the unique values in the table. Enter the following SQL statement:
SELECT DISTINCT state_id FROM supplsORDER BY state_id;| state_id |
|---|
| Russia |
| Ile de France |
| Pennsylvania |
| California |
| Washington |
| Michigan |
In this example, all unique state values are returned from the vendor table and all duplicates are removed from the result set. As you can see, the state of California in the results set is displayed only once, not four times.
SQL DISTINCT – search for unique values in multiple columns
Next let’s look at how to use SQL DISTINCT to remove duplicates from more than one field in a SELECT statement.
Using the same table from the previous example, enter the following SQL statement:
SELECT DISTINCT city_id, state_idFROM supplsORDER BY city_id, state_id;| city_id | state_id |
|---|---|
| Moscow | Russian |
| Lansing | Michigan |
| Redwood City | California |
| Redmond | Washington |
| Sunnyvale | Washington |
| Paoli | Pennsylvania |
| Paris | France |
| Menlo Park | California |
In this example, each unique combination of city and state will be returned. In this case, DISTINCT applies to each field specified after the DISTINCT keyword. As you can see, ‘Redwood City’, ‘California’ in the result set only appears once, not twice.
How DISTINCT handles NULL values
Finally, does the DISTINCT NULL operator consider it a unique value in SQL? The answer is yes. Let’s look at it further.
In this example, we have a product table with the following data:
| prod_id | prod_name | cat_id |
|---|---|---|
| 1 | Pear | 50 |
| 2 | Banana | 50 |
| 3 | Orange | 50 |
| 4 | Apple | 50 |
| 5 | Bread | 75 |
| 6 | Sliced Ham | 25 |
| 7 | Kleenex | NULL |
Now let us select unique values from the cat_id field, which contains the value NULL. Enter the following SQL query:
SELECT DISTINCT cat_idFROM prodsORDER BY cat_id;| cat_id |
|---|
| NULL |
| 25 |
| 50 |
| 75 |
In this example, the query will return the unique values found in the cat_id column. As you can see from the first line of the result set, NULL is the unique value returned by the DISTINCT operator.
Using DISTINCT in SQL
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