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This article mainly explains the "connection mode in sql". The explanation in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "connection mode in sql".

SQL connection can be divided into inner connection, outer connection and cross connection.

Database data:

Book table stu table

1. Internal connection

1.1. Equivalent join: use the equal sign (=) operator to compare the column values of the connected column in the join condition, and all columns in the joined table, including duplicate columns, are listed in the query results.

1.2. Unequal join: use a comparison operator other than the equal operator to compare the column values of the connected column in the join condition. These operators include >, > =,

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