mysql - Eliminate certain duplicated rows after group by -

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with db:

chef(cid,cname,age),

recipe(rid,rname),

cooked(orderid,cid,rid,price)

customers(cuid,orderid,time,daytime,age)

[cid means chef id, , on]

given orders customers, need find each chef, difference between age , average of people ordered his/her meals. wrote following query:

select cid, ch.age - avg(cu.age) diff chef ch natural join cooked co,customers cu co.orderid = cu.orderid group cid

this solves problem, if assume customers has unique id, might not work,because 1 can order 2 meals of same chef , affect calculation.

now know can answered not exists i'm looking soultion includes group function (something similar wrote). far couldn't find (i searched , tried many ways, select distinct , manipulation in clause ,to "having count(distinct..)" )

edit: people asked exmaple. i'm coding using sqlfiddle , crashes alot, i'll try best:

cid | cuid | orderid | cu.age ----------------------------- 1    333    1         20 1    200    2         41 1    200    5         41 2    4      3         36

let's chef 1's age 50 . query give 50 - (20+40+40/3) = 16 , 2/3. althought should 50 - (20+40/2) = 20. (because guy id 200 ordered 2 recipes of our beloved chef 1.). assume chef 2's age 47. query result:

cid | diff ---------- 1    16.667 2    11

another edit: wasn't taught particular sql-query form.so have no idea differences between oracle's mysql's microsoft server's, i'm "freestyle" querying.(i hope in exam :o )

first, should write query as:

select cid, ch.age - avg(cu.age) diff chef ch join      cooked co      on ch.cid = co.cid join      customers cu      on co.orderid = cu.orderid group cid;

two different reasons:

  • natural join bug waiting happen. list columns want used join, lest unexpected field or spelling difference affect results.
  • never use commas in from clause. use explicit join syntax.

next, answer question more complicated. each chef, can average age of customers doing:

select cid, avg(age)  (select distinct co.cid, cu.cuid, cu.age       cooked co join            customers cu            on co.orderid = cu.orderid      ) c group cid;

then, difference, need bring information in well. 1 method in subquery:

select cid, ( age - avg(cuage) ) diff (select distinct co.cid, cu.cuid, cu.age cuage, c.age cage       chef c join            cooked co            on ch.cid = co.cid join            customers cu            on co.orderid = cu.orderid      ) c group cid, cage;

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