尝试更新skill表sname列中的属性，但不允许更新，因为spossed和srequired表中存在外键约束。
这些外键也被声明为主键约束，因此不能为null。我该怎么办？
以下是表格：

CREATE TABLE SKILL(
sname         VARCHAR(30)     NOT NULL, /* Skill name */
CONSTRAINT SKILL_pkey PRIMARY KEY ( sname ) );

CREATE TABLE SREQUIRED(
sname         VARCHAR(30)     NOT NULL, /* Skill name*/
requires      VARCHAR(30)     NOT NULL, /* Skill required*/
slevel        DECIMAL(2)      NOT NULL, /* Level required*/
CONSTRAINT SREQUIRED_pkey PRIMARY KEY ( sname, requires ),
CONSTRAINT SREQUIRED_fkey1 FOREIGN KEY ( sname) REFERENCES SKILL( sname ),
CONSTRAINT SREQUIRED_fkey2 FOREIGN KEY ( requires ) REFERENCES SKILL( sname ) );

CREATE TABLE SPOSSESSED(
anumber     DECIMAL(6)  NOT NULL, /* Applicant number*/
sname       VARCHAR(30) NOT NULL, /* Skill name*/
slevel      DECIMAL(2)  NOT NULL, /* Skill level*/
CONSTRAINT SPOSSESSED_pkey PRIMARY KEY ( anumber, sname ), 
CONSTRAINT SPOSSESSED_fkey1 FOREIGN KEY ( anumber ) REFERENCES APPLICANT ( anumber ) ON DELETE CASCADE,
CONSTRAINT SPOSSESSED_fkey2 FOREIGN KEY ( sname )REFERENCES SKILL ( sname ),
CONSTRAINT SPOSSESSED_check1 CHECK ( slevel IN ( 1,2,3,4,5,6,7,8,9,10 ) ) );