Database query • Connect mind map & Essentials & Late (with examples)

Mind Mapping

1. Join: (table name) join (table name ) on ( table join relationship ), equivalent to the table to do a Cartesian product, and then filter.

For example

The results are as follows:

In a join, the order of each table is not in sequence

2. Left join: () leave Join () on () to keep the outer row

For example

This writing, will find that the results are missing, there is no professional college can not be found, need to use the left link

Results:

This makes the query complete.

For example:

in the College table, the professional table, the search for the establishment of 4-year professional school and its professional, non-qualified, non-professional colleges (departments) also listed

There is a missing query

and the following

You can complete the query

3. The self-join nature is the same as it is, just the joined table is its own

Such as

Database query • Connect mind map & Essentials & Late (with examples)