sql - TSQL Find Missing Values -

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i have 2 tables:

tablea calendar of possible weeks should have sales data per company.

tableb has financial data sales per company per week have been reported.

tablea columns 

[cmp_code, year, week]

tableb columns 

[cmp_code, year, week, sales]

query criteria:

1) list weeks missing tableb current year , previous year companies

2) list previous year sales value company , week if exists. (example cmp 1234 missing week 13, 2013, show value of week in 2012)

i've tried joins either 0 values returned or millions of values returned. don't know start.

i'm new sql , appreciate offered. in advance.

this 1 way:

select tablea.*, bprev.sales tablea left outer join tableb    on  tablea.cmp_code = tableb.cmp_code    , tablea.[year]   = tableb.[year]    , tablea.[week]   = tableb.[week] left outer join tableb bprev    on  tablea.cmp_code = bprev.cmp_code    , tablea.[year]   = bprev.[year]+1    , tablea.[week]   = bprev.[week] tableb.cmp_code null

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