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。