Here are 4 examples illustrating when you would use IF EXISTS and when you would use IF NOT ...
为了更好地演示NOT EXISTS的使用方法,下面给出一个完整的示例代码,包括创建表、插入数据和判断逻辑: CREATETABLEusers(idINTPRIMARYKEY,usernameVARCHAR(50),emailVARCHAR(100));INSERTINTOusers(id,username,email)VALUES(1,'john_doe','john.doe@example.com');INSERTINTOusers(id,username,email)SELECT2,'jane_s...
Here are 4 examples illustrating when you would use IF EXISTS and when you would use IF NOT ...
先看下面的例子:oracle中两个系统表.emp,dept. example: 1:not exists(not in) not exists: 这条语句返回select * from scott.dept d where e.deptno=d.deptno and d.deptno=10条件满足的结果集.也就是说, 返回的结果集中不存在d.deptno=10结果集的记录,即emp表中没有dept表中d.deptno=10的记录. SQ...
You can do this if the values you want to check in your main query exist in your database somewhere. Here’s an example, using the same data as the IN example. Let’s say we have another table that stores the names of food for the bakery section, called “bakery”. The “bakery”...
exists和notexists用法(Existsandnotexists) Exists(SQLreturnstheresultsetforreal) Notexists(SQLdoesnotreturntheresultset,true) Asfollows: TableA IDNAME 1A1 2A2 3A3 TableB IDAIDNAME 11B1 22B2 32B3 TableAandBis1tomanyrelationshipA.ID=>B.AID SELECT,ID,NAME,FROM,A,WHERE,EXIST(SELECT*FROM,B,WHE...
NOT EXISTS works like EXISTS, except the WHERE clause in which it is used is satisfied if no rows are returned by the subquery. For example, to find the names of products that are not in the wheels subcategory: USE AdventureWorks2008R2; GO SELECT Name FROM Production.Product WHERE NOT EXI...
NOT IN 和 <>ALL 对 NULL 值敏感,这意味着 NOT IN 后面的子查询或者常量集合一旦有 NULL 值出现,则整个 SQL 的执行结果就会为 NULL。 所以一旦相关的连接列上出现了 NULL 值(实际只会判断字段是否有 NOT NULL 约束),此时 Oracle 如果还按照通常的 ANTI JOIN 的处理逻辑来处理(实际和 INNER JOIN 的处理逻...
(SQL does not return the result set, true) As follows: Table A ID NAME 1 A1 2 A2 3 A3 Table B ID AID NAME 11 B1 22 B2 32 B3 Table A and B is 1 to many relationship A.ID = B.AID SELECT, ID, NAME, FROM, A, WHERE, EXIST (SELECT * FROM, B, WHERE, A.ID=B.AID) ...
In your example, the queries are semantically equivalent. In general, useEXISTSwhen: You don't need to return data from the related table You have dupes in the related table (JOINcan cause duplicate rows if values are repeated) You want to check existence (use instead ofLEFT OUTER JOIN.....