SELECTo.order_id,o.order_date,c.customer_nameFROMorders oJOINcustomers cONCASEWHENo.customer_id=1THENc.customer_id=o.customer_idWHENo.customer_id=2THENc.country='China'ELSEc.country='UK'END; 三、结论 掌握CASE 语句使我们能够直接在 SQL 查询中执行复杂的条件逻辑。无论对数据进行分类、对结果进...
We have constantly issues with different kinds of customers and based on their status or payment history, you want to join them to the loyalty tables. The focus was to come up with a solution that minimises the extra reads on the other tables but also to add this to a stored proc to m...
默认情况下,使用Join则代表Inner Join内连接,表示两个表根据某种等值规则进行连接。例如下面示例:查询所有学生的学号、姓名及所在班级 代码语言:javascript 代码运行次数:0 运行 AI代码解释 select p.Id,p.Name,c.Name from T_Person p join T_Class c on p.ClassId=c.Id 4.2 Left Join 例如:查询所有学生(...
然后我们观察到t2.date总是在case语句的两种情况下进行比较,即non-null。我们可以用下面的coalesce语句来表示case-statement: t2.date = coalesce(t1.date, t3date) 它看起来干净多了,但在功能上仍然与case语句相同。 请注意,如果您没有从t2和t3投射任何值,那么进行where date in (select date from t2) or...
CASE WHEN pref_name in ('德岛', '香川', '爱媛', '高知') THEN '九州' WHEN pref_name in ('福冈', '佐贺', '长崎') THEN '四国' ELSE '其他' END AS district FROM poptbl 然后再加载SUM()聚合函数求解每一个分类地区的人数。原文提到先进行GROUP BY再进行SELECT的方式在一些诸如Oracle、DB2、...
)ASp1INNERJOIN(SELECTpref_name,SUM(population)AScnt_fFROMPopTblWHEREsex='2'GROUPBYpref_name )ASp2ONp1.pref_name=p2.pref_name; 3、场景三:用 CHECK 约束定义多个列的条件关系 例:限定所有女性员工的工资不能高于 20 万 注:此处嵌套了 CASE 表达式,用于同时满足多种条件的时候;(如,此例中必须同时满...
join(region_population) nationwide = nationwide.rename(columns={'population': '全国'}) nationwide.reset_index() 输出如下: pandas的实现是这样的思路,首先对分组后的数据进行聚合操作,计算全国的总人口。然后按sex和pref_name列对数据进行分组,并计算各组内population列的总和,这会得到一个以sex和pref_name为...
方法一:where+not in遍历取无交集值并结合distinct去重selectdistinct Store_Name from location where(Store_Name)notin(select Store_Name from store_info);方法二:使用左连接(或者右连接)+where 判断NULL 取无交集并结合distinct去重selectdistinct A.Store_Name from location A leftjoinstore_info B using(Store...
SELECTCustomers.CustomerName,Orders.OrderIDINTOCustomersOrderBackup2017FROMCustomersLEFTJOINOrdersONCustomers.CustomerID=Orders.CustomerID; 1. 2. 3. 4. 提示:SELECT INTO还可以用于使用另一个表的架构创建新的空表。只需添加一个导致查询不返回数据的WHERE子句: ...
How to join to a CASE expression You can also join tables usingcase. For example, you may have a table of exam grades you want to link to the exam results. Use the case expression in the join clause like so: As withwhere, joining tocaselimits the optimizer. If you find yourself usin...