A FOREIGN KEY ensures data integrity by ensuring that data is entered into the database in accordance with certain standards. When you extract/query data from a database, you utilise a JOIN to specify how the data should be selected.
When you use SQL's JOIN function, it doesn't mean you can just join two tables:
Using a Junction Table to Connect Three Tables
The first step is to examine the schema and choose which columns to display.
The next step is to figure out which tables are required for the query.
We'll have to join all of the tables together at the end.
INNER, LEFT OUTER, RIGHT OUTER, FULL OUTER, and CROSS are the five forms of JOIN defined by the ANSI standard SQL.
In MySQL, the default JOIN is an INNER JOIN, which retrieves all rows from both tables based on the specified keywords as long as the columns in both tables match.
An INNER JOIN in SQL is the same as a JOIN clause in that it joins rows from two or more tables. The consequence of an inner join of A and B is A intersect B, or the inner half of a Venn diagram intersection.
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