Data

Tags

The tag operation has a variety of applications such as generating surrogate keys and finding the nth smallest or largest item.

Tag

The tag operator takes a relational expression, an ordered set of attributes and a tag attribute name. Each attribute name in the ordered set can be prefixed with - to use a descending order, otherwise an ascending order is used. An integer attribute is added to each tuple in the relational expression, incrementing within the common values of the ordered set.

Again, given the example SP relvar: 

SP
SNOPNOQTY


S1P1300


S1P2200


S1P3400


S1P4200


S1P5100


S1P6100


S2P1300


S2P2400


S3P2200


S4P2200


S4P4300


S4P5400

We can issue tags within the SNO (ascending) using the tag operator.

For example:

tag(SP, {SNO}, p_serial)
SNOPNOQTYp_serial


S1P13000


S1P22001


S1P34002


S1P42003


S1P51004


S1P61005


S2P13000


S2P24001


S3P22000


S4P22000


S4P43001


S4P54002

Nth smallest

We can tag the results of a quota operation and then filter it to find the nth smallest item. For example, the part(s) with the 2nd smallest weight:

tag(quota(P{WEIGHT}, {WEIGHT}, 2), {-WEIGHT}, t)[t=0] & P
WEIGHTtPNOPNAMECOLORCITY


140P4ScrewRedLondon

Nth largest

We can tag the results of a quota operation and then filter it to find the nth largest item. For example, the part(s) with the 2nd largest weight:

tag(quota(P{WEIGHT}, {-WEIGHT}, 2), {WEIGHT}, t)[t=0] & P
WEIGHTtPNOPNAMECOLORCITY


170P2BoltGreenParis


170P3ScrewBlueRome

These could be made into relational operators - see the Relation operators section for details.