(Cartesian Join) Cartesian connection: Each line of Table 1 is a record as a record with each line of Table 2. Such a query will return (number 1 line number) X (Table 2 Row number) record. There is nothing in such a connection but only shows that all connections are combined with the table. (Equi Join) Equal connection: Generally used columns with the same properties in the table. If there are employee tables and employee wages, there are employees in the two tables, and now you have to check all our employees' name, age, salary. From the Cartesian connection, you can take out the same line in the employee number of employees in the employee's table in the employee's table. The table is as follows:
Employee_tableemployee_pay_tableemployee_idemployee_idlast_namesalaryfirst_nameDepartmentMiddle_namesupervisor Marital_Status
The query statement can be written: select first_name, last_name, salary from Employee A, Employee_Pay B
Where a.employee_id = b.employee_ID
The result of this query contains all employees. If you want to query a specific employee, it is clear that you can get on this query, query statement is: select first_name, last_name, Salary from Employee A, Employee_Pay B
Where a.employee_id = b.employee_id and first_name = 'haha'
