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Primary index
A primary index is a file that has a sorted series of records having two columns: a block address for that key field in the data file; and the ordering key field. The ordering key field for this primary index can be the primary key of the data file. Primary index have one index entry for every value of the ordering key field. An entry in primary index file has the index value of the pointer to that data block and a first record of the data block.
Let us talk about primary index with the help of an instance. Let us suppose a student database as (Supposing that one block keeps only four student records.):
Enrolment Progra- Number Name City mme
BLOCK 1
2109348
ANU VERMA
CHENNAI
CIC
2109349
ABHISHEK KUMAR
CALCUTTA
MCA
2109351
VIMAL KISHOR
KOCHI
BCA
2109352
RANJEETA JULIE
BLOCK 2
2109353
MISS RAJIYA BANU
VARANASI
MBA
2238389
JITENDAR KASWAN
NEW DELHI
2238390
RITURAJ BHATI
2238411
AMIT KUMAR JAIN
BLOCK 3
2238412
PAWAN TIWARI
AJMER
2238414
SUPRIYA SWAMI
2238422
KAMLESH KUMAR
MUMBAI
BSC
2258014
DAVEN SINGHAL
BLOCK 4
2258015
S SRIVASTAVA
2258017
SHWETA SINGH
2258018
ASHISH TIWARI
2258019
SEEMA RANI
LUCKNOW
...
BLOCK r
2258616
NIDHI
2258617
JAGMEET SINGH
2258618
PRADEEP KUMAR
2318935
RAMADHAR
FARIDABAD
BLOCK N-1
2401407
BRIJMISHRA
BAREILLY
2401408
AMIT KUMAR
2401409
MD. IMRAN SAIFI
AURANGABAD
2401623
ARUN KUMAR
BLOCK N
2401666
ABHISHEK RAJPUT
2409216
TANNUJ SETHI
2409217
SANTOSH KUMAR
ALMORA
2409422
SAKSHI GINOTRA
Figure: A Student file stored in the order of student enrolment numbers
While sorting field groups we cannot use more than one key field False.
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