Data Query connection

Data Query connection

Part 1 Definition

A join query is a query connection composed of a Cartesian Product Operation plus a selection operation. It means that two datasets are merged horizontally to generate a new result set.

Part 2 Classification

1.The main feature is to delete all the rows that are not matched in other connected tables from the results, so the inner join will lose data information.

2 external connections. External connections expand the internal connection function, but some data from the table source will be deleted.

A left Outer Join.Retain all rows in the first table and the rows matching the first table in the second table.Empty rows are placed in null values.

B. Right outer join.All rows in the second table are retained, including the rows matching the first table and the second table.Empty rows are placed in null values.

CAll external connections. All rows are displayed in the result table. The rows that do not meet the conditions are placed in the null value.

3. Cross join. In the cross join operation, each row in the first table is matched with each row in the second table, which leads to all possible merging.

Column in The crosstab join = sum of the number of columns in the original table (SUM)

The row in the cross join = the product of the number of rows in the original table (multiplied)