Compare current row value with previous row values in loop in SQL

Question

I have a table with fields - A (ID), B (Flag). I need to add a new column - C (Result) in this table and it's value will be derived based on B (Flag) field. If flag is false then keep checking previous rows till we get flag as true and then take value of A (ID) field and populate it in C (Result) column. So C will have the last value of A with B field as True.

Required Table

A	B	C
1	T	1
2	F	1
3	F	1
4	T	4
5	T	5
6	F	5
7	T	7
8	F	7
9	F	7
10	F	7
11	T	11

Answer 1

WITH
cte1 AS (
    SELECT A, SUM(B='T') OVER (ORDER BY A) group_no
    FROM test
),
cte2 AS (
    SELECT A, MIN(A) OVER (PARTITION BY group_no) previous_T
    FROM cte1
)
UPDATE test
JOIN cte2 USING (A)
SET test.C = cte2.previous_T;

https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=46c1edfbc42e331f9bb39a36ea71e905

In MySQL 5.x use

UPDATE test
JOIN (
    SELECT A, 
           @tmp := CASE WHEN B='T' THEN A ELSE @tmp END C
    FROM test
    JOIN (SELECT @tmp:=0) init
    ORDER BY A
) data USING (A)
SET test.C = data.C;

https://dbfiddle.uk/?rdbms=mysql_5.7&fiddle=3f341b98b768c2a2369b5d814453b234