sql語言演示!

lifan5748發表於2005-08-26

1.在查詢結果中顯示列名:

  a.用as關鍵字:select name as '姓名' from students order by age

  b.直接表示:select name '姓名' from students order by age

  2.精確查詢:

  a.用in限定範圍:select * from students where native in ('湖南', '四川')

  b.between...and:select * from students where age between 20 and 30

  c.“=”:select * from students where name = '李山'

  d.like:select * from students where name like '李%' (注意查詢條件中有“%”,則說明是部分匹配,而且還有先後資訊在裡面,即查詢以“李”開頭的匹配項。所以若查詢有“李”的所有物件,應該命令:'%李%';若是第二個字為李,則應為'_李%'或'_李'或'_李_'。)

  e.[]匹配檢查符:select * from courses where cno like '[AC]%' (表示或的關係,與"in(...)"類似,而且"[]"可以表示範圍,如:select * from courses where cno like '[A-C]%')

  3.對於時間型別變數的處理

  a.smalldatetime:直接按照字串處理的方式進行處理,例如:
select * from students where birth > = '1980-1-1' and birth <= '1980-12-31'

  4.集函式

  a.count()求和,如:select count(*) from students (求學生總人數)

  b.avg(列)求平均,如:select avg(mark) from grades where cno=’B2’

  c.max(列)和min(列),求最大與最小

  5.分組group

  常用於統計時,如分組查總數:
select gender,count(sno)
from students
group by gender
(檢視男女學生各有多少)

  注意:從哪種角度分組就從哪列"group by"

  對於多重分組,只需將分組規則羅列。比如查詢各屆各專業的男女同學人數 ,那麼分組規則有:屆別(grade)、專業(mno)和性別(gender),所以有"group by grade, mno, gender"

select grade, mno, gender, count(*)
from students
group by grade, mno, gender

  通常group還和having聯用,比如查詢1門課以上不及格的學生,則按學號(sno)分類有:

select sno,count(*) from grades
where mark<60
group by sno
having count(*)>1

  6.UNION聯合

  合併查詢結果,如:

SELECT * FROM students
WHERE name like ‘張%’
UNION [ALL]
SELECT * FROM students
WHERE name like ‘李%’

  7.多表查詢

  a.內連線

select g.sno,s.name,c.coursename
from grades g JOIN students s ON g.sno=s.sno
JOIN courses c ON g.cno=c.cno
(注意可以引用別名)
b.外連線
b1.左連線
select courses.cno,max(coursename),count(sno)
from courses LEFT JOIN grades ON courses.cno=grades.cno
group by courses.cno

  左連線特點:顯示全部左邊表中的所有專案,即使其中有些項中的資料未填寫完全。

  左外連線返回那些存在於左表而右表中卻沒有的行,再加上內連線的行。

  b2.右連線

  與左連線類似

  b3.全連線

select sno,name,major
from students FULL JOIN majors ON students.mno=majors.mno

  兩邊表中的內容全部顯示

  c.自身連線

select c1.cno,c1.coursename,c1.pno,c2.coursename
from courses c1,courses c2 where c1.pno=c2.cno

  採用別名解決問題。

  d.交叉連線

select lastname+firstname from lastname CROSS JOIN firstanme

  相當於做笛卡兒積

  8.巢狀查詢

  a.用關鍵字IN,如查詢李山的同鄉:

select * from students
where native in (select native from students where name=’ 李山’)

  b.使用關鍵字EXIST,比如,下面兩句是等價的:

select * from students
where sno in (select sno from grades where cno=’B2’)

select * from students where exists
(select * from grades where
grades.sno=students.sno AND cno=’B2’)

  9.關於排序order

  a.對於排序order,有兩種方法:asc升序和desc降序

  b.對於排序order,可以按照查詢條件中的某項排列,而且這項可用數字表示,如:

select sno,count(*) ,avg(mark) from grades
group by sno
having avg(mark)>85
order by 3

  10.其他

  a.對於有空格的識別名稱,應該用"[]"括住。

  b.對於某列中沒有資料的特定查詢可以用null判斷,如select sno,courseno from grades where mark IS NULL

  c.注意區分在巢狀查詢中使用的any與all的區別,any相當於邏輯運算“||”而all則相當於邏輯運算“&&”

  d.注意在做否定意義的查詢是小心進入陷阱:

  如,沒有選修‘B2’課程的學生 :

select students.*
from students, grades
where students.sno=grades.sno
AND grades.cno <> ’B2’

  上面的查詢方式是錯誤的,正確方式見下方:

select * from students
where not exists (select * from grades
where grades.sno=students.sno AND cno='B2')

  11.關於有難度多重巢狀查詢的解決思想:

  如,選修了全部課程的學生:

select *
from students
where not exists ( select *
from courses
where NOT EXISTS
(select *
from grades
where sno=students.sno
AND cno=courses.cno))

  最外一重:從學生表中選,排除那些有課沒選的。用not exist。由於討論物件是課程,所以第二重查詢從course表中找,排除那些選了課的即可。

SQL>delete cz where (c1,c10,c20) in (select c1,c10,c20 from cz group by c1,c10,c20 having count(*)>1) and rowid not in
(select min(rowid) from cz group by c1,c10,c20 having count(*)>1);

SQL>delete cz where rowid not in(select min(rowid) from cz group by c1,c10,c20);


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