用同一行的值更新表

Question

I have a table like this

id     Name   Parent_id   Text
1        x      0         
2        y      1         
3        Z      1        
4        A      2         
5        B      0         

Now i'd like to populate the column with the text of the column name depending on parent_id. If parent_id is 0 text value will be the name of the same row instead id parent_id!=0 it must find the right name

this is the output I'd like to archieve

id     Name   Parent_id   Text
1        x      0         x
2        y      1         x
3        z      1         x
4        a      2         y
5        b      0         b

Could someone help me?

Answer 1

First, write a SELECT statement that returns the row, along with the value you want to assign. Something like this:

SELECT t.id                            AS t_id
     , t.parent_id                     AS t_parent_id
     , t.name                          AS t_name
     , p.id                            AS p_id
     , p.name                          AS p_name
     , t.text                          AS t_text_old
     , IF(t.parent_id=0,t.name,p.name) AS t_text_new
  FROM tlt t
  LEFT
  JOIN tlt p
    ON p.id = t.parent_id

Not all of the columns in the SELECT list are necessary, but I find it helpful to examine the results to confirm that the statement is correct.

Once you get the SELECT statement working, returning the new value(s) you want to assign to the column(s), convert that into an UPDATE statement.

Replace the SELECT ... FROM with the keyword UPDATE , and add a SET clause immediatly before the WHERE clause (or at the end of the statement if there isn't a WHERE clause.

UPDATE tlt t
  LEFT
  JOIN tlt p
    ON p.id = t.parent_id
   SET t.text = IF(t.parent_id=0,t.name,p.name)

Answer 2

Try:

UPDATE tablename t
SET Text = 
      CASE 
           WHEN t.Parent_id = 0 THEN t.Name
           ELSE ( SELECT t1.name FROM tablename t1
                  WHERE t1.id = t.parent_id )
      END;