我们假设我有一本餐桌杂志:
CREATE TABLE magazine
(
magazine_id integer NOT NULL DEFAULT nextval(('public.magazine_magazine_id_seq'::text)::regclass),
longname character varying(1000),
shortname character varying(200),
issn character varying(9),
CONSTRAINT pk_magazine PRIMARY KEY (magazine_id)
);
另一个表格问题:
CREATE TABLE issue
(
issue_id integer NOT NULL DEFAULT nextval(('public.issue_issue_id_seq'::text)::regclass),
number integer,
year integer,
volume integer,
fk_magazine_id integer,
CONSTRAINT pk_issue PRIMARY KEY (issue_id),
CONSTRAINT fk_magazine_id FOREIGN KEY (fk_magazine_id)
REFERENCES magazine (magazine_id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION
);
当前INSERTS:
INSERT INTO magazine (longname,shotname,issn)
VALUES ('a long name','ee','1111-2222');
INSERT INTO issue (fk_magazine_id,number,year,volume)
VALUES (currval('magazine_magazine_id_seq'),'8','1982','6');
现在只应在“杂志”中插入一行,如果它尚不存在的话。但是,如果它存在,则表'issue'需要获取已存在的行的'magazine_id'以便建立引用。
我该怎么做?
提前谢谢!答案 0 :(得分:1)
你怎么知道一本杂志是否已经在magazine
表中? issn
列是否定义了杂志?如果是,那么它应该是主键,或者至少是unique
。
最简单的方法是在客户端应用程序中检查杂志是否存在,如下所示(伪代码):
function insert_issue(longname, shotname, issn, number,year,volume) {
/* extensive comments for newbies */
start_transaction();
q_get_magazine_id = prepare_query(
'select magazine_id from magazine where issn=?'
);
magazine_id = execute_query(q_get_magazine_id, issn);
/* if magazine_id is null now then there’s no magazine with this issn */
/* and we have to add it */
if ( magazine_id == NULL ) {
q_insert_magazine = prepare_query(
'insert into magazine (longname, shotname, issn)
values (?,?,?) returning magazine_id'
);
magazine_id = execute_query(q_insert_magazine, longname, shortname, issn);
/* we have tried to add a new magazine; */
/* if we failed (magazine_id==NULL) then somebody else just added it */
if ( magazine_id == NULL ) {
/* other, parerelly connected client just inserted this magazine, */
/* this is unlikely but possible */
rollback();
start_transaction();
magazine_id = execute_query(q_get_magazine_id, issn);
}
}
/* now magazine_id is an id of magazine, */
/* added if it was not in a database before, new otherwise */
q_insert_issue = prepare_query(
'insert into issue (fk_magazine_id,number,year,volume)
values (?,?,?,?)'
);
execute_query(q_insert_issue, magazine_id, number, year, volume);
/* we have inserted a new issue referencing old, */
/* or if it was needed new, magazine */
if ( ! commit() ) {
rollback();
raise "Unable to insert an issue";
}
}
如果您只需要在一个查询中执行此操作,那么您可以将此伪代码实现为数据库中的pl / pgsql函数,而只是select insert_issue(?, ?, ?, ?, ?, ?)
。
答案 1 :(得分:0)
如果您使用PostgreSQL 9.1或更高版本,则可以执行以下操作:
WITH ref_key (id) AS (
WITH ins (id) AS (
INSERT INTO magazine (longname,shotname,issn)
VALUES ('a long name','ee','1111-2222')
RETURNING id
)
SELECT id
FROM magazine
LEFT JOIN ins USING id
WHERE issn = '1111-2222'
)
INSERT INTO INTO issue (fk_magazine_id,number,year,volume)
SELECT id,'8','1982','6'
FROM ref_key;
可写CTE的FTW!
答案 2 :(得分:-1)
我不确定你是否可以用SQL做到这一点。我知道Oracle可以用于触发器,但我不认为SQL能够。如果我错了,有人会纠正我。