A Cartesian product
SELECT * FROM table 1, table 2
Traverse the records of two tables to display
Horizontal connection of two tables
1 using foreign key relationships as a condition
SELECT * from table 1, table 2 where table 1 table 2 foreign KEY constraint relationship
Select column 1, column 2, from table 1, table 2 where table 1 foreign KEY constraint relationship for table 2
2 Join on
SELECT * FROM table 1 join table 2 on table 1 table 2 foreign KEY constraint relationship
Select column 1, column 2 from table 1 join table 2 on table 1 foreign KEY constraint relationship for table 2
The LEFT JOIN table is the main table when it joins
The right join table is connected with the table as the main --draw one record with one table to determine the combination with another table, the same duplicate extraction, no display null
Vertical connection of three tables
Union
Select column from table 1 Union Select column from table 2
--The columns to be concatenated must have the same column name, and the data types are equal
SQL Server T-SQL table connections