Trigger Function → Selecting a unique row using a values in a none unique column - sql

Question

I got a sticky situation here whereby I setup a trigger to update a table (Self-updating function). What I got here is that the function is able to identify that there is an update operation however, it cannot located the row to update as there are no unique value in the column.
TRIG_NS_ABS4_To_Area_func (Trigger Func):
BEGIN
IF (TG_OP = 'UPDATE') AND (OLD."ABS4" <> NEW."ABS4")
THEN UPDATE systems."NS_HandoverReportInput_tbl" SET ("Area") = ((SELECT "NS_AREA" FROM systems."NS_ABS4Area Match_tbl" WHERE "NS_ABS4" = NEW."ABS4"))
WHERE "NSItemNumber" = NEW."NSItemNumber";
END IF;
RETURN NEW;
END;
I was wondering whether does anyone have any idea to locate the row to update.
Please bear in mind that ONLY the "NSItemNumber" field is unique else the rest of the fields may have repeating values.
Script:
CREATE TABLE systems."NS_HandoverReportInput_tbl" (
"NSItemNumber" SERIAL,
"ABS4" TEXT,
"Area" TEXT,
CONSTRAINT "PK_NS_HandoverReportInput_tbl" PRIMARY KEY("NSItemNumber"),
)
WITH (oids = false);
CREATE TRIGGER "NS_ABS4Area Match_tbl"
AFTER INSERT OR UPDATE
ON systems."NS_HandoverReportInput_tbl" FOR EACH ROW
EXECUTE PROCEDURE systems."TRIG_NS_ABS4_To_Area_func"();
NS_ABS4Area Match_tbl display info as listed:
NSItemNumber | ABS4 | Area
1001 | AAAA |Toilet
1002 | AABB |Central Area
1003 | AACC |Carpark
1004 | AAAA |Toilet
1005 | AABB |Central Area

Answer 1

I'll give you two solutions, a working one and a good one.
The working solution
Use a BEFORE INSERT OR UPDATE trigger, then you don't have to locate the row to update, because instead of changing the table you change the values before they are written to the table.
You could define your trigger like this:
CREATE OR REPLACE FUNCTION "TRIG_NS_ABS4_To_Area_func"() RETURNS trigger LANGUAGE plpgsql AS
$$BEGIN
-- this will fail if there is more than one "NS_AREA" per "NS_ABS4"
SELECT DISTINCT "NS_AREA" INTO STRICT NEW."NS_AREA"
FROM "NS_ABS4Area Match_tbl"
WHERE "NS_ABS4" = NEW."ABS4";
RETURN NEW;
END;$$;
CREATE TRIGGER "NS_ABS4Area Match_tbl"
BEFORE INSERT OR UPDATE
ON systems."NS_HandoverReportInput_tbl" FOR EACH ROW
EXECUTE PROCEDURE systems."TRIG_NS_ABS4_To_Area_func"();
The good solution
You avoid the whole mess by normalizing your database design.
That way no inconsistencies can ever happen, and you don't need a trigger.
CREATE TABLE area_description (
abs4 text PRIMARY KEY,
area text NOT NULL
);
COPY area_description FROM STDIN (FORMAT 'csv');
'AAAA', 'Toilet'
'AABB', 'Central Area'
'AACC', 'Carpark'
\.
CREATE TABLE ns_report_input (
ns_item_number serial PRIMARY KEY,
abs4 text REFERENCES area_description(abs4)
);
CREATE INDEX ns_report_input_fkex_ind ON ns_report_input(abs4);
You can define a view if you want something that looks like your original table.