correct way to create a pivot table in postgresql using CASE WHEN
case-when, pivot, pivot-table, postgresql, sql
Solution
Use MAX() or MIN(), not FIRST(). In this scenario, you will have all NULLs in the column per each group value except for, at most, one with a not null value. By definition, this is both the MIN and the MAX of that set of values (all nulls are excluded).
Problem
I am trying to create a pivot table type view in postgresql and am nearly there! Here is the basic query: ``` select acc2tax_node.acc, tax_node.name, tax_node.rank from tax_node, acc2tax_node where tax_node.taxid=acc2tax_node.taxid and acc2tax_node.acc='AJ012531'; ``` And the data: ``` acc | name | rank ----------+-------------------------+-------------- AJ012531 | Paromalostomum fusculum | species AJ012531 | Paromalostomum | genus AJ012531 | Macrostomidae | family AJ012531 | Macrostomida | order AJ012531 | Macrostomorpha | no rank AJ012531 | Turbellaria | class AJ012531 | Platyhelminthes | phylum AJ012531 | Acoelomata | no rank AJ012531 | Bilateria | no rank AJ012531 | Eumetazoa | no rank AJ012531 | Metazoa | kingdom AJ012531 | Fungi/Metazoa group | no rank AJ012531 | Eukaryota | superkingdom AJ012531 | cellular organisms | no rank ``` What I am trying to get is the following: ``` acc | species | phylum AJ012531 | Paromalostomum fusculum | Platyhelminthes ``` I am trying to do this with CASE WHEN, so I've got as far as the following: ``` select acc2tax_node.acc, CASE tax_node.rank WHEN 'species' THEN tax_node.name ELSE NULL END as species, CASE tax_node.rank WHEN 'phylum' THEN tax_node.name ELSE NULL END as phylum from tax_node, acc2tax_node where tax_node.taxid=acc2tax_node.taxid and acc2tax_node.acc='AJ012531'; ``` Which gives me the output: ``` acc | species | phylum ----------+-------------------------+----------------- AJ012531 | Paromalostomum fusculum | AJ012531 | | AJ012531 | | AJ012531 | | AJ012531 | | AJ012531 | | AJ012531 | | Platyhelminthes AJ012531 | | AJ012531 | | AJ012531 | | AJ012531 | | AJ012531 | | AJ012531 | | AJ012531 | | ``` Now I know that I have to group by acc at some point, so I try ``` select acc2tax_node.acc, CASE tax_node.rank WHEN 'species' THEN tax_node.name ELSE NULL END as sp, CASE tax_node.rank WHEN 'phylum' THEN tax_node.name ELSE NULL END as ph from tax_node, acc2tax_node where tax_node.taxid=acc2tax_node.taxid and acc2tax_node.acc='AJ012531' group by acc2tax_node.acc; ``` But I get the dreaded ``` ERROR: column "tax_node.rank" must appear in the GROUP BY clause or be used in an aggregate function ``` All the previous examples I've been able to find use something like SUM() around the CASE statements, so I guess that is the aggregate function. I have tried using FIRST(): ``` select acc2tax_node.acc, FIRST(CASE tax_node.rank WHEN 'species' THEN tax_node.name ELSE NULL END) as sp, FIRST(CASE tax_node.rank WHEN 'phylum' THEN tax_node.name ELSE NULL END) as ph from tax_node, acc2tax_node where tax_node.taxid=acc2tax_node.taxid and acc2tax_node.acc='AJ012531' group by acc2tax_node.acc; ``` but get the error: ``` ERROR: function first(character varying) does not exist ``` Can anyone offer any hints?