How to set MYSQL column to certain value if a certain row exists?

Question

I want to generate a new column in my table that is true if a row exists with certain conditions.

name | col1 | col2 | flag
--------------------------
a      1           


        

Answer 1

So you want just to get those values from db? or you want to add column? those are 2 different goals.

So if you need just to get those values you can:

http://sqlfiddle.com/#!9/65b4c2/1

SELECT t.*, t2.flag
FROM table_name t
LEFT JOIN (
  SELECT name, MAX(IF(col1=2 AND col2=3,1,0)) flag
  FROM table_name
  GROUP BY name
  ) t2
ON t.name = t2.name

and if you really need to add new column then you go this way:

http://sqlfiddle.com/#!9/226fb3/1

ALTER TABLE table_name ADD COLUMN flag TINYINT;

UPDATE table_name t
LEFT JOIN (
      SELECT name, MAX(IF(col1=2 AND col2=3,1,0)) flag
      FROM table_name
      GROUP BY name
      ) t2
ON t.name = t2.name
SET t.flag=t2.flag