北美数据科学SQL面经（JOIN）

What is a table join in SQL?

A table join in SQL combines rows from two or more tables based on a related column between them. It allows you to retrieve data from multiple tables as if they were a single table.

What are the different types of table joins?

The different types of table joins in SQL are:

• INNER JOIN: Returns only the matching rows between the tables.

• LEFT JOIN: Returns all the rows from the left table and the matching rows from the right table.

• RIGHT JOIN: Returns all the rows from the right table and the matching rows from the left table.

• FULL OUTER JOIN: Returns all the rows from both the left and right tables, including unmatched rows.

What is the difference between INNER JOIN and OUTER JOIN?

INNER JOIN only returns the matching rows between the tables, excluding unmatched rows. OUTER JOIN, on the other hand, includes unmatched rows from one or both tables in the result set.

How do you perform an INNER JOIN in SQL?

To perform an INNER JOIN in SQL, you specify the join condition using the ON keyword. Here's an example:

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SELECT *
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FROM table1
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INNER JOIN table2 ON table1.column = table2.column;

How do you perform a LEFT JOIN in SQL?

To perform a LEFT JOIN in SQL, you use the LEFT JOIN keyword and specify the join condition. Here's an example:

xxxxxxxxxx
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SELECT *
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FROM table1
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LEFT JOIN table2 ON table1.column = table2.column;

How do you perform a RIGHT JOIN in SQL?

To perform a RIGHT JOIN in SQL, you use the RIGHT JOIN keyword and specify the join condition. Here's an example:

xxxxxxxxxx
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SELECT *
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FROM table1
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RIGHT JOIN table2 ON table1.column = table2.column;

How do you perform a FULL OUTER JOIN in SQL?

SQL does not have a standard FULL OUTER JOIN syntax in all database systems. However, you can achieve a similar result using a combination of LEFT JOIN and RIGHT JOIN with UNION. Here's an example:

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SELECT *
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FROM table1
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LEFT JOIN table2 ON table1.column = table2.column
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UNION
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SELECT *
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FROM table1
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RIGHT JOIN table2 ON table1.column = table2.column;

Can you have multiple table joins in a single query?

Yes, you can have multiple table joins in a single query. You can join multiple tables by specifying additional JOIN clauses and join conditions.

How do you specify the join conditions in a table join?

You specify the join conditions in a table join by using the ON keyword followed by the condition that relates the columns between the tables. For example:

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SELECT *
2
FROM table1
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INNER JOIN table2 ON table1.column = table2.column;

What is the difference between ON and WHERE clauses in a table join?

The ON clause is used to specify the join condition, which determines how the tables are related and which rows should be combined. The WHERE clause, on the other hand, is used to filter the result set based on conditions unrelated to the join itself. The ON clause is specific to the join operation, while the WHERE clause applies to the entire result set.