Is there a faster alternative to insert into X (select Y from where Z) in mysql?

Victor :

table_a

user_id  score  
1        10     
2        10      
3        10      
5        43      
6        43      
9        20      
10       42    



table_b

user_id  flag
1        0
2        0
3        0
4        1
5        1
6        0 
7        1

In the above case, how can I populate the table_b.flag to 1 if user_id from table_a has a (score > 40)?

Nick :

You can use a multi-table UPDATE to get the results you want:

UPDATE table_b b
JOIN table_a a ON a.user_id = b.user_id AND a.score > 40
SET b.flag = 1

The JOIN condition means that only rows in table_b where the corresponding user_id in table_a has a score > 40 will have their flag set to 1. If you want to also set flags to 0 if the corresponding score <= 40, you can use the condition as the value to SET (since MySQL treats booleans as 1 or 0 in a numeric context):

UPDATE table_b b
JOIN table_a a ON a.user_id = b.user_id
SET b.flag = a.score > 40

For your sample data, the result in table_b is the same:

user_id     flag
1           0
2           0
3           0
4           1
5           1
6           1
7           1

Demo on dbfiddle

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