SQL
\ SQL.
.
, , , .
α , (), (), () , , . :
) ,
, D; , D, a D; D ;
) ,
, θ - , , D.
, R , R, (R WHERE α), , :
R WHERE α(r) = {r | r R α (r) = ""}
() .
, :
:
;
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: , , .
1. , .
2. ? *.
3. : (=), (>), (<), (>=), (<=), (< >)), (not >), (not <).
. (AND).
2.
1.1.78 - 1.1.83.
:
_2 = ( WHERE ([] ="") &
([_]> 1.01.78) & ([_] < 1.01.83))[_]
, , , _. : .
QBE :
_2
_ | _ | _ | ||
ü | ||||
"" | >#1.1.78# | <#1.1.83# |
SQL:
|
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