const:仅仅能查询到一条数据的sql,用于primary key 或unique索引(类型与索引类型有关) eq_ref:唯一性索引: 对于每个索引键的查询,返回匹配唯一行数据(有且只有一个,不能多,不能0) 常见于唯一索引和主键索 引: ex:alter table teacherCard add constraint pk_id primary key(id) alter table teacher add cons...
'sql',2);insertintocoursevalues(4,'web',3);insertintoteachervalues(1,'tz',1);insertintoteachervalues(2,'tw',2);insertintoteachervalues(3,'tl',3);insertintoteacherCardvalues(1,'tzdesc');insertintoteacherCardvalues(2,'twdesc');insertintoteacherCardvalues(3,'tldesc');...
添加唯一键语法:alter table 表名 add constraint 索引名 unique index(列名) 检查字段是否唯一键:show index form 表名;被展示出来的皆是有唯一约束的; 以上级别,均是可遇不可求!!! 四.ref级别 到ref还是问题不大的,只要你上点心,就可以达到; 非唯一性索引:对于每个索引键的查询,返回匹配的所有行(可以是0...
id值有相同,又有不同:值越大越优先;id值相同,从上往下 顺序执行 -- 查询教授SQL课程的老师的描述(desc) mysql>explainselectcourse.cname,teacher.tname,teacherCard.tcdescfromcourse,teacher,teacherCardwherecourse.tid=teacher.tidandteacher.tcid=teacherCard.tcidandcourse.cname='sql'; +---+---+---+-...
SELECT识别符,每个select语句都会自动分配的一个唯一标识符。SQL执行的顺序的标识。遵循以下原则 id从大到小的执行 id相同时,执行顺序由上至下 id列为null表示为结果集,不需要使用这个语句来查询 2-1.id相同 查询演员id为1的电影信息 explain select * from film f where f.film_id in (select film_Id from...
CONSTRAINT`fk_payment_rental`FOREIGN KEY(`rental_id`)REFERENCES`rental`(`rental_id`)ON DELETE SET NULL ON UPDATE CASCADE,CONSTRAINT`fk_payment_staff`FOREIGN KEY(`staff_id`)REFERENCES`staff`(`staff_id`)ON UPDATE CASCADE)ENGINE=InnoDB AUTO_INCREMENT=16050DEFAULT CHARSET=utf81row in set(0.00sec...
* constraint name unless VERBOSE is specified. In non-text formats * we just print everything. */ if (es->format == EXPLAIN_FORMAT_TEXT) { if (es->verbose || conname == NULL) appendStringInfo(es->str, "Trigger %s", trig->tgname); else appendStringInfoString(es->str, "Trigger"...
CONSTRAINT `FK_DEPTNO` FOREIGN KEY (`DEPTNO`) REFERENCES `dept` (`DEPTNO`)) ENGINE=InnoDB DEFAULT CHARSET=utf8;-- --- -- Records of emp -- --- INSERT INTO `emp` VALUES ('7369', 'SMITH', 'CLERK', '7902', '1980-12-17', '800', null, '20');INSERT INTO `emp` VALUES ('74...
CREATE TABLE public.course (id int8 NOT NULL GENERATED BY DEFAULT AS IDENTITY,language_id int8 NOT NULL,"name" varchar(100) NOT NULL,"level" int4 NULL,created_date timestamp NULL,"version" varchar(50) NULL,total_lessons int4 NULL,country varchar(100) NOT NULL,CONSTRAINT course_pkey PRIMA...
1.正在使用所有可用的并行工作进程（max_parallel_workers）。这是一种可能的解释，因为即使使用您显示的...