GBase 8s V8. 8 SQL Guide: Tutorial - 6.2.1 (1)

6.2.1 Self join  
The join does not always involve two different tables , You can connect a table to itself , Create self connection . When you want to put in a column
When the value of is compared with other values in the same column , It's useful to connect a table to itself .
To create a self connection , stay FROM The table is listed twice in the clause , And give it a different alias each time . Use the alias
stay Projection and WHERE Reference table in clause . It's like two separate tables .（SELECT In the sentence
Alias in alias and GBase 8s SQL guide ： Discussion in grammar .）
The same as the connection between tables , You can use arithmetic expressions in a self join , You can test null values . have access to ORDER 
BY Clause to sort the values in the specified column in ascending or descending order .
The following query ship_weight Five times or more and ship_date Orders that are not empty . next , Search by
mirror ship_date Sort the data .
chart : Inquire about
SELECT x.order_num, x.ship_weight, x.ship_date,
 y.order_num, y.ship_weight, y.ship_date 
 FROM orders x, orders y
 WHERE x.ship_weight >= 5 * y.ship_weight
 AND x.ship_date IS NOT NULL
 AND y.ship_date IS NOT NULL
 ORDER BY x.ship_date;