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Coalesce

Building on the vein of representing one-to-many relationships I've always been frustrated trying to display child records in tables and datagrids. Examples include showing multiple authors of publications, children in a classroom, and fish species observed in a lake.

Until today the two best methods I had were both hacks.
Method 1
Use Three Queries
1.) Build a crosstab
2.) Concatenate the output of the crosstab
3.) Join the product back to the parent table

- or -

Method 2
Loop through child records programmatically to build a comma-delimited list.

Today I discovered a far more elegant solution using COALESCE within a User Defined Function to build comma-delimited list in Views and Stored Procedures.

For this example we'll be building a comma-delimited list of fish species observed in each water (lakes and streams).

Step 1: Create a new User Defined Function(UDF)
CREATE FUNCTION dbo.udf_GetFishByWater(@WaterID int)
RETURNS VARCHAR(1000) AS

BEGIN
DECLARE @FishList varchar(1000)

SELECT @FishList = COALESCE(@FishList + ', ', '') + SpeciesName
FROM VU_FishSpeciesByWater
WHERE WaterID = @WaterID

RETURN @FishList
END


Step 2: Call the User Defined Function in a View
SELECT WaterName As Stream, Parent,
dbo.udf_GetFishByWater(WaterID) as FishSurveyed
FROM GIS_Hydrography


Result:
StreamParentFishSurveyed
Abbott GulchS. Fk. Boise RiverRedbank Trout, Sculpin (Var. Species)
Adair CreekJungle CreekCutthroat Trout, Westslope Cutthroat Trout
Adams CreekMann CreekRainbow / Redband Trout (Wild), Rainbow Trout
Agency CreekLemhi RiverBull Trout, Cutthroat Trout, Rainbow Trout, Sculpin (Var. Species)
Ahrs GulchSaint Joe RiverBrook Trout, Cutthroat Trout
Airplane LakeShip Island CreekCutthroat Trout, Rainbow Trout
Alder CreekMorgan CreekCutthroat Trout, Rainbow / Redband Trout (Wild), Rainbow X Cutthroat Trout
Alder CreekPatterson CreekBull Trout, Cutthroat Trout, Rainbow / Redband Trout (Wild), Rainbow X Cutthroat Trout, Unknown Species
Alder CreekS. Fk. Payette RiverDace (Var. Sp.), Rainbow Trout, Sucker (Var. Sp.)
...

Link

NOTE: These are now stored as Scalar-valued Functions in SQL Server 2008

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