用MySQL中另一张表中的数据更新一列中的许多行

Question

Suppose I have a table1 like below:

CREATE TABLE IF NOT EXISTS table1
(
table1_id INT NOT NULL, 
field1 INT DEFAULT NULL, 
field2 INT DEFAULT NULL,

FOREIGN KEY (field1) REFERENCES fields(field_id),
FOREIGN KEY (field2) REFERENCES fields(field_id),
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

and a table2:

CREATE TABLE IF NOT EXISTS table2
(
table2_id INT NOT NULL,
another_id INT NOT NULL,
field1 INT DEFAULT NULL,

PRIMARY KEY (table2_id , another_id),
FOREIGN KEY (another_id) REFERENCES table1(table1_id),
FOREIGN KEY (field1) REFERENCES fields(field_id),
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

I have filled with data the columns table1_id , field1 , field2 of table1 and table2_id , another_id of table2.

What I want to do is fill with data the field1 column of table2. The thing is I want that to be either value field1 from table1 or field2 from table1. I want that to be random. If though one of the them is NULL select the other, if both are, just put NULL.

I can simply create a few UPDATES queries and do it right?. Unfortunately I cant cause the table2 has more than 5k rows and I can do 5k UPDATE queries. Moreover am not exactly sure of how to make the MySQL choose at random one of the two fields of table1 as described above. Let me give you below an example query of how it would be.

(I dont know how to select at random columns and with conditions so I'll leave that out in my query - Suppose that out of the random condition the result was to select field1)

UPDATE table2 SET table2.field1 = (SELECT table1.field1 FROM table1 WHERE table1_id = another_id) WHERE another_id = X;

I'd have to repeat the above for many different X's.

So the question is how to fill the field1 of table2 with either field1 or field2 from table1 with the conditions described above for many X values?

Answer 1

To randomly select field1 or field2 , or whichever one is not NULL :

UPDATE table2 SET table2.field1 = (
    SELECT
        CASE
            WHEN ( table1.field1 IS NULL AND table1.field2 IS NULL ) THEN NULL
            WHEN ( table1.field1 IS NULL ) THEN table1.field2
            WHEN ( table1.field2 IS NULL ) THEN table1.field1
            ELSE IF( RAND() < 0.5, table1.field1, table1.field2 )
        END CASE
    FROM table1 WHERE table1_id = another_id
);