How to reload SQL

There are four ways to deduplicate in SQL: 1. DISTINCT keyword: eliminates duplicate values ​​in a single column. 2. GROUP BY clause: Group by column and return unique rows. 3. Subquery: Select a unique value from the main query. 4. Collection operator: merge result sets and eliminate duplicate values.

Deduplication method in SQL

How to use SQL to deduplicate?

There are several ways to deduplication in SQL, including:

1. DISTINCT keywords

• Use the DISTINCT keyword to eliminate duplicate values ​​from the result set.
• Syntax: SELECT DISTINCT column_name(s) FROM table_name;

2. GROUP BY clause

• The GROUP BY clause groups the result set and returns only a unique row in each group.
• Syntax: SELECT column_name(s) FROM table_name GROUP BY column_name(s);

3. Subquery

• Use a subquery to select a unique value from the main query.
• Syntax: SELECT column_name(s) FROM table_name WHERE column_name IN (SELECT DISTINCT column_name FROM table_name);

4. Collection operator

• The set operator UNION combines two result sets and eliminates duplicate values.
• Syntax: SELECT column_name(s) FROM table_name UNION SELECT column_name(s) FROM table_name2;

Detailed example:

DISTINCT keywords:

 <code class="sql">SELECT DISTINCT name FROM customers;</code>

GROUP BY clause:

 <code class="sql">SELECT name FROM customers GROUP BY name;</code>

Subquery:

 <code class="sql">SELECT name FROM customers WHERE name IN (SELECT DISTINCT name FROM customers);</code>

Collection operator:

 <code class="sql">SELECT name FROM customers UNION SELECT name FROM orders;</code>

Choose the best method:

• If you only need to eliminate duplicate values ​​from a single column, you can use the DISTINCT keyword.
• If you need to group by multiple columns, you can use the GROUP BY clause.
• If you need to select a unique value from a subquery, you can use the subquery.
• If you need to merge two or more result sets and eliminate duplicate values, you can use the collection operator.

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