mysql修改表以添加选定的列

Question

I am trying to use ALTER to add a new column (authorID) to the tests table, but I want to have a specific value (which I don't know).

I am trying to run the following, but getting SQL errors. Is this possible, if so where have I gone wrong please?

SELECT id FROM authors WHERE emailAddress = 'UNASSIGNED' into @unassignedID;
ALTER TABLE Tests ADD COLUMN authorID INT NOT NULL DEFAULT @unassignedID;

Authors table:

CREATE TABLE authors
(
  ID INT NOT NULL AUTO_INCREMENT,
  emailAddress VARCHAR(100) NOT NULL UNIQUE,
  regionID INT NOT NULL,
  PRIMARY KEY(ID)
);

Tests Tables:

CREATE TABLE Tests
(
  testID INT NOT NULL AUTO_INCREMENT,
  Title VARCHAR(100) DEFAULT NULL, 
  PRIMARY KEY(testID)
);

Answer 1

you can use prepared statement :

SELECT id into @unassignedID   FROM authors  WHERE emailAddress = 'UNASSIGNED' limit 1; 
SET @sql = CONCAT('ALTER TABLE Tests  ADD COLUMN authorID INT NOT NULL DEFAULT ',@unassignedID);
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;