SQLクエリの個別のキーワードの目的は何ですか？

DISTINCT关键字在SQL中用于去除查询结果中的重复行。其核心作用是确保返回的每一行数据都是唯一的，适用于获取单列或多列的唯一值列表，如部门、状态或名称等。使用时需注意DISTINCT作用于整行而非单列，且常与多列组合使用时返回所有列的唯一组合。基本语法为SELECT DISTINCT column_name FROM table_name，可应用于单列或多列查询。使用时需注意其性能影响，尤其是在大数据集上需进行排序或哈希操作。常见误区包括误以为DISTINCT仅作用于单列、在无需去重的场景下滥用DISTINCT，以及用其掩盖错误的JOIN或分组逻辑。正确做法应是理解并解决数据重复的根本原因。

The DISTINCT keyword in SQL is used to eliminate duplicate rows from the result set of a query. When you run a query that returns data, sometimes the same row can appear multiple times due to how the data is structured or joined. Using DISTINCT ensures that each row in the result is unique.

When to Use DISTINCT

You typically use DISTINCT when you want to get a list of unique values from one or more columns. This is especially useful when dealing with columns that might have repeated entries like categories, statuses, or names.

For example:

• Getting a list of all unique departments from an employees table.
• Listing all different job titles without repetition.

It’s important to note that DISTINCT applies to the entire row returned, not just a single column. So if you're selecting multiple columns, it will return combinations that are unique across all those columns.

How DISTINCT Works in Practice

Using DISTINCT is straightforward — just place it after the SELECT keyword:

SELECT DISTINCT department FROM employees;

This query will return a list of all departments where at least one employee works, with no duplicates.

If you're selecting from multiple columns:

SELECT DISTINCT department, location FROM employees;

This will return unique combinations of department and location.

Some things to keep in mind:

• DISTINCT can affect performance, especially on large datasets, because it requires sorting or hashing to remove duplicates.
• It's often used alongside aggregate functions like COUNT() to avoid counting duplicates.

Common Mistakes with DISTINCT

A common mistake is assuming that DISTINCT removes duplicates only in one column while ignoring others. For instance:

SELECT DISTINCT name, email FROM users;

This will return unique combinations of name and email, not just unique names.

Another pitfall is using DISTINCT unnecessarily. If you know your data doesn’t contain duplicates (like querying a primary key), then DISTINCT isn't needed and just adds overhead.

Also, some people try to "overuse" DISTINCT as a fix-all for messy joins or incorrect groupings. While it may hide duplicates temporarily, it's better to understand why duplicates are appearing and address the root cause — maybe the join logic or aggregation is off.

So, use DISTINCT when you need to ensure uniqueness in your results, but don’t rely on it to fix deeper issues in your query structure.

基本上就这些。

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