What are the uses of left join?

Usage: 1. LEFT JOIN ON: Use the ON clause to specify the join condition; 2. LEFT JOIN USING: Use the USING clause to specify the join condition, which is a column with the same name in the two tables; 3 , LEFT OUTER JOIN: LEFT OUTER JOIN is a synonym for LEFT JOIN, which is used to return all records in the left table and the records in the right table that meet the join conditions; 4. LEFT JOIN multiple tables, etc.

LEFT JOIN is a SQL statement used to join records from two or more tables. LEFT JOIN returns all records in the left table and records in the right table that meet the join conditions. If there are no records that meet the condition in the right table, NULL is returned.

The usage of LEFT JOIN is as follows:

1. LEFT JOIN ON: Use the ON clause to specify the connection conditions.

For example: SELECT * FROM table 1 LEFT JOIN table 2 ON table 1. column = table 2. column;

2. LEFT JOIN USING: Use the USING clause to specify the join condition. This condition is a column with the same name in both tables.

For example: SELECT * FROM Table 1 LEFT JOIN Table 2 USING (column);

3. LEFT OUTER JOIN: LEFT OUTER JOIN is a synonym for LEFT JOIN and is used to return the values ​​in the left table All records, as well as records in the right table that meet the join conditions.

For example: SELECT * FROM table 1 LEFT OUTER JOIN table 2 ON table 1. column = table 2. column;

4. LEFT JOIN multiple tables: multiple LEFT JOIN connections can be used Multiple tables.

For example: SELECT * FROM Table 1 LEFT JOIN Table 2 ON Table 1. Column = Table 2. Column LEFT JOIN Table 3 ON Table 1. Column = Table 3. Column;

Need to pay attention to What is important is that the use of LEFT JOIN needs to be determined based on specific business requirements and data table structure to ensure that the connection conditions are correct and the results are as expected.

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