Using the distinct keyword in parsing SQL

Detailed explanation of distinct usage in SQL

In SQL databases, we often encounter situations where we need to remove duplicate data. At this time, we can use the distinct keyword, which can help us remove duplicate data and make the query results clearer and more accurate.

The basic usage of distinct is very simple, just use the distinct keyword in the select statement. For example, the following is an ordinary select statement:

SELECT column_name FROM table_name;

If we want to remove duplicate data, just add the distinct keyword in front of column_name:

SELECT DISTINCT column_name FROM table_name;

The distinct keyword will be used to column_name Deduplicate the data and return the deduplicated results.

The distinct keyword can be used not only for deduplication of single column data, but also for deduplication of multi-column data. For example, suppose we have a table named orders, which contains two fields: order number (order_id) and customer name (customer_name). If we want to remove duplicate order numbers and customer names, we can use the following query statement:

SELECT DISTINCT order_id, customer_name FROM orders;

The above statement will return the result set after removing duplicate order numbers and customer names.

It should be noted that the distinct keyword applies to the entire result set, not just a single column. That is, it removes duplicate rows from the entire result set. Therefore, when using the distinct keyword, we need to ensure that the data sets being compared are consistent. For example, the following is an incorrect query statement:

SELECT DISTINCT column_name1, column_name2 FROM table_name;

The above query statement is incorrect because when comparing two columns, the values ​​of both columns must be considered simultaneously and duplicate rows must be removed. Otherwise, the deduplication effect will not be achieved.

In some cases, we may encounter a situation where columns need to be sorted. The distinct keyword can be used with the order by clause to remove duplicate data in a specific order. For example, the following is an example:

SELECT DISTINCT column_name FROM table_name ORDER BY column_name ASC/DESC;

The above statement will return a deduplication result set sorted by the column_name column. ASC means ascending order, DESC means descending order.

In addition to the basic distinct usage, we can also use the distinct keyword to perform some other operations. For example, we can use the count function to count the number of result rows after the distinct keyword is removed. For example:

SELECT COUNT(DISTINCT column_name) FROM table_name;

The above statement will return the number of unique rows in the result set after deduplication.

In actual use, we can also use other SQL operations in combination with the distinct keyword, such as table joins, subqueries, conditional filtering, etc. For example, the following is an example of combining distinct and subquery:

SELECT DISTINCT column_name FROM table_name WHERE column_name IN (SELECT column_name FROM table_name WHERE condition);

The above statement will return unique column_name values ​​that meet the conditions.

To sum up, the distinct keyword is a commonly used keyword in SQL and is used to deduplicate query results. By removing duplicate data, we are able to get a clearer and more accurate result set. When using distinct, you need to note that the columns or combinations being compared need to be consistent, and it can also be combined with other SQL operations for more complex query result processing. I hope this article can provide a more detailed understanding of distinct usage.

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