Subquery Practice One: Find out all information about people older than 35 years in the sales department
Exercise two: Convert all numeric codes from a department in a haha table to a string display in a bumen table
Exercise three: Convert all the number codes from the Department in the haha table to the string display in the Bumen table, and show each person's supervisor
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, and then query through the connection.
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:
Database Basics (subquery exercises, link queries (join on, union) and their exercises)