update from select

 CREATE TABLE dualx(
        x_id VARCHAR(5) NOT NULL ,
        x_con VARCHAR(10)
 )  

 CREATE TABLE dualy(
        y_id VARCHAR(5) NOT NULL ,
        y_con VARCHAR(10)
 )  

 INSERT INTO dualx VALUES ('','xxx');
 INSERT INTO dualx VALUES ('','xxx');
 INSERT INTO dualx VALUES ('','xxx');  

 INSERT INTO dualy VALUES ('','');
 INSERT INTO dualy VALUES ('','');
 INSERT INTO dualy VALUES ('','');  

 UPDATE dualx   SET (x_con) = (SELECT y.y_con  FROM  dualy y WHERE dualx.x_id = y.y_id)  

 SELECT * FROM dualx;
 SELECT * FROM dualy;
 DROP TABLE dualx;
 DROP TABLE dualy;  

UPDATE dualx SET (x_con) = (SELECT y.y_con FROM dualy y WHERE dualx.x_id = y.y_id) 其中SELECT y.y_con FROM dualy y WHERE dualx.x_id = y.y_id不能返回多行，否则报错。