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 Procedural SQL (PSQL) StatementsAggregate Functions 

NULLIF Example

select avg( nullif(Weight, -1) ) from FatPeople

This will return the average weight of the persons listed in FatPeople, excluding those having a weight of -1, since AVG skips NULL data.Presumably, -1 indicates “weight unknown” in this table.A plain AVG(Weight) would include the -1 weights, thus skewing the result.

See also

[fblangref50-scalarfuncs-coalesce], [fblangref50-scalarfuncs-decode], [fblangref50-scalarfuncs-iif], CASE