Question

I have two tables:

CREATE TABLE Table1 (
    ID VARCHAR(15),
    Value INT
);
INSERT INTO Table1 (`ID`, `Value`)
  VALUES
('A', 21),
('B', 50),
('C', 10);

CREATE TABLE Table2 (
    ID VARCHAR(15)
);
INSERT INTO Table2 SELECT ID FROM Table1;

I have to rename all the IDs in Table2 whose values in table1 < 20 to “Low_<ID Name>”. After this, “Table2” should look like:

mysql> SELECT ID FROM Table2
+-------+
| ID    |
+-------+
| A     |
| B     |
| Low_C |
+-------+

What query will do this?

Answer 1

UPDATE Table2
LEFT JOIN Table1 ON Table2.ID=Table1.ID
SET Table2.ID = CONCAT('Low_',Table2.ID)
WHERE Table1.Value<20

But as @Phil said…are you sure you want to change your (presumably) primary key ID?