Unique Record Selection in SQL
The keyword in SQL is a powerful tool for filtering unique data from a table, either from a single column or multiple columns. This article will explain how to use and provide examples to help you understand its functionality.
Using DISTINCT with single and multiple columns
To get unique values from a single column, use the following syntax:
For example, to get unique customer names from an table:
To get unique combinations across multiple columns, use the following syntax:
For example, to get unique pairs of customer-product combinations from an table:
When you specify multiple columns with , the result will include rows where the combination of values in those columns is distinct. This means duplicate rows where all the specified column values match will be removed, but rows with unique combinations in those columns will be retained.
Examples
Single column example
Given a table with columns and , the following query returns unique customer names:
This query removes duplicates and shows only unique customer names. For example, if "Karan" appears multiple times, it shows only once.
Multiple columns example
Using the same table:
This returns unique pairs of customer-product combinations. For example, even if "Karan" ordered "Laptop" multiple times, that pair appears only once, but if "Karan" ordered "Phone" as well, that would appear as a separate row.
Additional notes
- works on the entire row of the specified columns, not on individual columns separately.
- You can combine with other clauses such as to organize the results.
- To count distinct combinations, use to get the number of unique pairs.
Sample table and data for clarity
```sql CREATE TABLE orders ( order_id INT, customer_name VARCHAR(50), product_name VARCHAR(50) );
INSERT INTO orders VALUES (1, 'Karan', 'Laptop'), (2, 'Yuva', 'Phone'), (3, 'Karan', 'Laptop'), (4, 'Yuva', 'Tablet'), (5, 'Karan', 'Phone'); ```
This sample data demonstrates the use of the keyword in action.
Conclusion
In summary, the keyword is a valuable tool for filtering unique data, either from one or several columns, by returning rows where the combination of specified column values is unique. It is an important tool for ensuring data integrity and precision, especially during data analysis and cleaning. By understanding the keyword and its applications, you can streamline your data analysis and manipulation tasks in SQL.
Technology in data-and-cloud-computing, such as SQL, provides the 'DISTINCT' keyword, a powerful tool for filtering unique data from a table, either from a single column or multiple columns. In the context of multiple columns, a 'trie' (implied as a data structure that efficiently stores and retrieves unique combinations) can be used for fast retrieval of unique combinations across multiple columns, as demonstrated in the examples provided. For example, a query using 'DISTINCT' on multiple columns can be optimized using a trie data structure to retrieve unique pairs of customer-product combinations from a table quickly and efficiently.
