SQL Database Fundamentals (eight)

Connection query: You can implement multiple table queries by using the Join operator. Connection is the main feature of relational database model, and it is also a sign that distinguishes it from other types of database management system.

Common two link operators:

1.join on

2.union

in the relational database management system, the relationship between the data is not determined, and all the information of an entity is often stored in a single table. When retrieving data, the connection operation queries the information for different entities that reside in multiple tables. The connection operation gives the user a lot of flexibility and they can add new data types at any time. Create a new table for different entities, followed by a connection for querying.

Example one:

Example two:

Example three:

Example four:

Example five:

Example SIX:

Example Seven:

Example eight:

A data query between three tables:

Practice One:

Exercise two:

Exercise Three:

Exercise Four:

Exercise Five:

SQL Database Fundamentals (eight)