//查出Category 表中categoryID不等于n.categoryID的c.categoryID,c.[name] 第一种查询语句:select c.categoryID,c.[name] from Category c left join News n on c.categoryID=n.categoryID where n.categoryID is null 第二种查询语句:select categoryID,[name] from category where not exists ( select ...
10.exists的用法 //查出Category 表中categoryID不等于n.categoryID的c.categoryID,c.[name] 第一种查询语句:select c.categoryID,c.[name] from Category c left join News n on c.categoryID=n.categoryID where n.categoryID is null 第二种查询语句:select categoryID,[name] from category where not ...
importpymysql# 连接到MySQL数据库conn=pymysql.connect(host='localhost',port=3306,user='root',password='password',db='your_database')# 编写SQL语句sql="SELECT EXISTS(SELECT 1 FROM your_table WHERE your_condition)"# 执行SQL语句cursor=conn.cursor()cursor.execute(sql)result=cursor.fetchone()[0]...
结论:student2 大表在内适用 exists,所以第 2 条 SQL 比第 1 条快;student1 小表在内适用 in,所以第 3 条 SQL 比第 4 条快。 not in 与 not exists 的查询 SQL AI检测代码解析 select count(1) from student1 where sname not in (select sname from student2); select count(1) from student1 ...
SQL 複製 DROP TABLE IF EXISTS #SampleTempTable; GO CREATE TABLE #SampleTempTable (id INT, message nvarchar(50)); INSERT INTO #SampleTempTable VALUES (null, 'hello') ; INSERT INTO #SampleTempTable VALUES (10, null); INSERT INTO #SampleTempTable VALUES (17, 'abc'); INSERT INTO #Sample...
SQL DROPTABLEIFEXISTS#SampleTempTable; GOCREATETABLE#SampleTempTable (idINT, messagenvarchar(50));INSERTINTO#SampleTempTableVALUES(null,'hello') ;INSERTINTO#SampleTempTableVALUES(10,null);INSERTINTO#SampleTempTableVALUES(17,'abc');INSERTINTO#SampleTempTableVALUES(17,'yes');INSERTINTO#SampleTempTableV...
To add a database to an availability group, the database must be an online, read-write database that exists on the server instance that hosts the primary replica. When you add a database, it joins the availability group as a primary database, while remaining available to clients. No corr...
To add a database to an availability group, the database must be an online, read-write database that exists on the server instance that hosts the primary replica. When you add a database, it joins the availability group as a primary database, while remaining available to clients. No corr...
所以LEFT JOIN...IS NULL和NOT EXISTS二者对于重复数据一个通过两部操作完成先完全JOIN后进行过滤,而另外一个则是直接通过右半联接过滤。所以对于此二者最大的不同在于:当使用LEFT JOIN...IS NULL时,SQL还没有那么聪明,仅仅只检查一次,因此它需要通过完全JOIN和过滤来完成,而NOT EXISTS则是在JOIN时就进行过滤。
col_x,1)); --11g版本后not in和not exists趋于相似,也可以用not in --当t.col_x不存在等于1的数据时等价于 --select*from tab_i t where t.col_x is not null; 比较大小 代码语言:javascript 代码运行次数:0 运行 AI代码解释 --当t.col_x为总是大于1的数值时 select*from tab_i t where 1...