删除sybase 12.5中的外键

I have two tables:

我有两张桌子:

CREATE TABLE dbo.country
(
cntry_id  VARCHAR(2)    NOT NULL,
name      VARCHAR(50)   NOT NULL,

CONSTRAINT pk_country PRIMARY KEY (cntry_id)

CREATE TABLE dbo.city
(
city_id   VARCHAR(3)    NOT NULL,
name      VARCHAR(50)   NOT NULL,
cntry_id  VARCHAR(2)    NOT NULL,

CONSTRAINT pk_city PRIMARY KEY (city_id),
FOREIGN KEY (cntry_id) REFERENCES dbo.country(cntry_id)
) 
CREATE TABLE dbo