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...
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';+---+---+---+---...
INDEX`salary`(`salary`) USING BTREE,INDEX`name_sal_did`(`name`, `salary`, `department_id`) USING BTREE,INDEX`name_did`(`name`, `department_id`) USING BTREE,CONSTRAINT`employees_ibfk_1`FOREIGNKEY(`department_id`)REFERENCES`departments` (`id`)ONDELETERESTRICTON...
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...
` char(3) NOT NULL DEFAULT '', `District` char(20) NOT NULL DEFAULT '', `Population` int NOT NULL DEFAULT '0', PRIMARY KEY (`ID`), KEY `CountryCode` (`CountryCode`), CONSTRAINT `city_ibfk_1` FOREIGN KEY (`CountryCode`) REFERENCES `country` (`Code`) ) ENGINE=InnoDB AUTO_...
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...
Advanced SQL SQL - Wildcards SQL - Injection SQL - Hosting SQL - Min & Max SQL - Null Functions SQL - Check Constraint SQL - Default Constraint SQL - Stored Procedures SQL - NULL Values SQL - Transactions SQL - Sub Queries SQL - Handling Duplicates SQL - Using Sequences SQL - Auto Inc...
* 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"...
add constraint FK_TEST_1 foreign key (FID) references HEK_TEST_IN (PID); create index HEK_TEST_INDETAIL_INDEX on HEK_TEST_INDETAIL (NNAME, NSIZE); 2.测试具体SQL语句到底有没有使用index。 2.1条件查询: select * from hek_test_in where pid=3 ...
alter table teacherCard add constraint pk_tcid primary key(tcid); 由于外键在语法上允许重复,而eq_ref需要数据唯一,所以也给teacher中tcid加上唯一索引: alter table teacher add constraint uk_tcid unique index(tcid); explain select t.tcid from teacher t,teacherCard tc where t.tcid = tc.tcid; mysq...