Dab tsi yog rename lag luam hauv DBMS?
Dab tsi yog rename lag luam hauv DBMS?

Video: Dab tsi yog rename lag luam hauv DBMS?

Video: Dab tsi yog rename lag luam hauv DBMS?
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Hloov npe ua haujlwm (ρ)

Cov txiaj ntsig ntawm kev sib raug zoo algebra kuj yog kev sib raug zoo tab sis tsis muaj npe. Cov rename kev ua haujlwm tso cai rau peb hloov npe cov zis kev sib raug zoo. ' hloov npe ' kev ua haujlwm yog denoted nrog me Greek tsab ntawv rho ρ.

Kuj paub, qhov projection ua haujlwm hauv DBMS yog dab tsi?

Nyob rau hauv relational algebra, a qhov projection yog unary kev ua haujlwm sau li. qhov twg. yog ib lub npe ntawm tus cwj pwm. Qhov tshwm sim ntawm xws li qhov projection yog txhais raws li lub teeb tau txais thaum lub Cheebtsam ntawm lub tuple yog txwv rau lub teeb. - nws muab pov tseg (lossis tsis suav nrog) lwm tus cwj pwm.

Tsis tas li ntawd, dab tsi yog qhov kev sib raug zoo algebra ua haujlwm txhawb nqa hauv SQL? Relational algebra tsuas yog muab lub hauv paus theoretical rau kev sib txheeb databases thiab SQL.

  • Cov neeg ua haujlwm hauv Relational Algebra.
  • Qhov projection (π)
  • Nco tseg: Los ntawm Default projection tshem tawm cov ntaub ntawv duplicate.
  • Kev xaiv (σ)
  • Nco tseg: xaiv tus neeg teb xov tooj tsuas yog xaiv cov tuples uas yuav tsum tau tab sis tsis tso rau lawv.
  • Union (U)
  • Teeb qhov txawv (-)

Dab tsi yog qhov txawv ntawm kev xaiv thiab qhov project ua piv txwv?

PROJECT tshem tawm cov kab thaum Xaiv tshem tawm cov kab. Xaiv retrieves cov tuples (kab) hauv a kev sib raug zoo (cov lus) uas cov xwm txheej hauv 'predicate' seem (Qhov twg clause) sawv qhov tseeb. Qhov project retrieves tus cwj pwm (kem) teev.

Tus yuam sij txawv teb chaws hauv DBMS yog dab tsi?

A txawv teb chaws key yog ib kem lossis pab pawg kab hauv cov rooj sib tham database uas muab kev sib txuas ntawm cov ntaub ntawv hauv ob lub rooj. Lub tswvyim ntawm referential kev ncaj ncees yog muab los ntawm txawv teb chaws key kev xav. Cov yuam sij txawv teb chaws thiab lawv qhov kev siv yog qhov nyuaj tshaj qhov tseem ceeb yuam sij.

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