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2 ÊÂÎñµÄ4´óÌØÐÔ
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3.select count£¨*£© from student ºÍselect count£¨id£© from student Ö®¼äµÄÇø±ð¡£
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select count(*) ͳ¼ÆËùÓÐѧÉúµÄ¼Ç¼¸öÊý£¬°üÀ¨¿Õ¼Ç¼¡£
Select count(Id) ͳ¼ÆËùÓÐѧÉúµÄ¼Ç¼¸öÊý£¬²»°üÀ¨null¼Ç¼¡£
4¼ÙÉèÏÖÔÚÓбísystem.table1£¬±íÖÐÓÐÈý¸ö×ֶΣºid(ÊýÖµÐÍ)¡¢name£¨×Ö·ûÐÍ£©¡¢age£¨ÊýÖµÐÍ£©Ð´³öSQLÓï¾äÍê³ÉÈçϹ¦ÄÜ£ºÔÚ±íÖвé³öÄêÁä´óÓÚ20£¬ÇÒÃû×ÖÒÔ¡°Íõ¡±¿ªÍ·µÄ¼Ç¼£¬²¢ÇÒ°´ÕÕÄêÁäµÄµ¹ÐðÅÅÁгöÀ´£¨ÄêÁä´óµÄÔÚÇ°Ã棩¡£
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Select * from system.table1 where age>20 and name like ¡®Íõ%¡¯ order by age DESC;
5 .´´½¨CUSTOMERS±í£¬×Ö¶ÎΪ£ºID£º£¨·Ç¿Õ£¬Ö÷¼ü£©bigint£¬NAME£º£¨·Ç¿Õ£©varchar£¬AGE£ºintÀàÐÍ£»´´½¨ORDERS±í£¬×Ö¶ÎΪ£ºID£º£¨·Ç¿Õ£¬Ö÷¼ü£¬£©bigint£¬ORDER_NUMBER£º£¨·Ç¿Õ£©varchar£¬PRICE£ºdouble£¬CUSTOMER_ID £º£¨Íâ¼ü£©bigint£¬ÉèÖü¶Á¬É¾³ý£»
´ð°¸£ºcreate table CUSTOMBERS(
ID bigint not null,
NAME varchar(15),
AGE int,
primary key (ID)
);
create table ORDERS(
ID bigint not null,
ORDER_NUMBER varchar(15) not nulll,
PRICE double precision,
CUSTOMER_ID bigint,
primary key(ID),
);
alter table ORDERS add constraint FK_CUSTOMER foreign key (CUSTOMER_ID) references CUSTOMERS(ID) on delete cascade;
6.ʹÓÃ×óÍâÁ¬½Ó²éѯ£¬ORDERS ºÍ CUSTOMERS ±í£¬
´ð°¸£ºselect c.ID, o.CUSTOMER_ID,c.NAME, o.ID ORDER_ID,ORDER_NUMBER from CUSTOMERS c left outer join ORDERS o no c.ID=o.CUSTOMER_ID;
29 .¼òÊöÊý¾Ý¿âÊÂÎñµÄÉúÃüÖÜÆÚ£¿£¨¿É»Á÷³Ìͼ£©
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7.delete from tablea & truncate table tableaµÄÇø±ð
truncate Óï¾äÖ´ÐÐËٶȿì,Õ¼×ÊÔ´ÉÙ,²¢ÇÒÖ»¼Ç¼ҳɾ³ýµÄÈÕÖ¾£»
delete ¶ÔÿÌõ¼Ç¼µÄɾ³ý¾ùÐèÒª¼Ç¼ÈÕÖ¾