How to conditionally set value for mysql row group? [duplicate] - mysql

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 2 0
a 2 3 0
b 1 2 0
b 4 3 0
Lets say I want to set the flag to 1 for every name identifier if a row exists with that name and where col1=2 and col2 = 3. So this would result in:
name | col1 | col2 | flag
--------------------------
a 1 2 1
a 2 3 1
b 1 2 0
b 4 3 0
because for a a row with col1=2 and col2 = 3 exists, but for b, such a row doesn't exist.
In pseudocode I want something like this:
ALTER TABLE table_name
ADD flag TINYINT(1)
IF ##row with condition col1=value1 and col2=value2 exists#
GROUP BY name
How can I generate this column?

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