Postgres的SQL查询

Question

I am trying to write a query from one table name 't'. It has mainly 4 columns involved in my query. From this sample data, I need a specific output.

Unit_name      Unit_id_from   unit_transferred_to     Action
------------------------------------------------------------
UNITABC        011102X         0215478Y               CONVERTED  
UNITQWE        0222487Y        NULL                   NEW RAISED   
UNITASDF       0215478Y        NULL                   INVALID   
UNITPOU        0487985Z        08975469K              CONVERTED  
UNITHUT        08975469K       NULL                   INACTIVE

The name of unit_transferred_to is actually in first column unit_name. So I need name of the unit_transferred_to from unit_name and the action = 'converted'

The output needs to look like this:

Unit_name   Unit_id_from   unit_transferred_to     unit_transfer_name     Action
----------------------------------------------------------------------------------
UNITABC       011102X        0215478Y              UNITASDF               CONVERTED  
UNITPOU       0487985Z       08975469K             UNITHUT                CONVERTED

Answer 1

This seems like a join :

select ti.Unit_name, ti.Unit_id_from, ti.unit_transferred_to,
       tito.unit_name as unit_transfer_name, 
       ti.Action
from ti join
     ti tito
     on ti.unit_transferred_to = tito.unit_transferred_from
where ti.action = 'CONVERTED'