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B.External connection is divided into left connection, right connection and self-connection.
C.The horizontal part of a database table is called a row, the vertical part is called a column, the content of each row is called a record,and the column name of each column is called a fiel
D.Before operating a data table,you should specify in which database the operation is performed using the "USE database name", otherwise the "No database selected" error will be thrown.