约束数据库表，使一列中只有一行可以具有特定值

Question

我是 SQL 的新手，当我尝试遵循 here 帖子中的建议以在我的专栏中只执行一个“是”时，我试图找出一条错误消息。

DROP TABLE team  CASCADE CONSTRAINTS PURGE;
create table team (
  name varchar2(4) NOT NULL UNIQUE,
  isTeamLead char(3) 
    check (isTeamLead in ('Yes')));

create unique index only_one_yes on team(isTeamLead)
(case when col='YES' then 'YES' end);

报错如下：

Error report -
SQL Error: ORA-02158: invalid CREATE INDEX option
02158. 00000 -  "invalid CREATE INDEX option"
*Cause:    An option other than COMPRESS, NOCOMPRESS, PCTFREE, INITRANS,
           MAXTRANS, STORAGE, TABLESPACE, PARALLEL, NOPARALLEL, RECOVERABLE,
           UNRECOVERABLE, LOGGING, NOLOGGING, LOCAL, or GLOBAL was specified.
*Action:   Choose one of the valid CREATE INDEX options.

有什么想法吗？

运行 Oracle Database 11g Enterprise Edition Release 11.2.0.1.0

Answer 1

删除create index 语句的case 部分。这执行OK：

create table team (
  name varchar2(4) NOT NULL UNIQUE,
  isTeamLead char(3) check (isTeamLead in ('Yes'))
);

create unique index only_one_yes on team(isTeamLead);

insert into team values ('x', 'Yes');
insert into team values ('y', null);

见SQLFiddle。