PostgreSQL - check if column exists and nest condition statement - sql

Question

Transactions column's names in below code are dynamicaly generated (so it means that sometimes particular name/column doesn't exist). Using this select it finishes successfully only in case when every of those names exists, if not, I got error like this (example):
Error(s), warning(s): 42703: column "TransactionA" does not exist
SELECT
*,
((CASE WHEN "TransactionA" IS NULL THEN 0 ELSE "TransactionA" END) -
(CASE WHEN "TransactionB" IS NULL THEN 0 ELSE "TransactionB" END) +
(CASE WHEN "TransactionC" IS NULL THEN 0 ELSE "TransactionC" END)) AS "Account_balance"
FROM Summary ORDER BY id;
Could you tell me please how can I check first if the column exists and then how can I nest another CASE statement or other condition statement to make it working in a correct way?

Answer 1

You can build any query dynamically with information from the Postgres catalog tables. pg_attribute in your case. Alternatively, use the information schema. See:
Query to return output column names and data types of a query, table or view
How to check if a table exists in a given schema
Basic query to see which of the given columns exist in a given table:
SELECT attname
FROM pg_attribute a
WHERE attrelid = 'public.summary'::regclass -- tbl here
AND NOT attisdropped
AND attnum > 0
AND attname IN ('TransactionA', 'TransactionB', 'TransactionC'); -- columns here
Building on this, you can have Postgres generate your whole query. While being at it, look up whether columns are defined NOT NULL, in which case they don't need COALESCE:
CREATE OR REPLACE FUNCTION f_build_query(_tbl regclass, _columns json)
RETURNS text AS
$func$
DECLARE
_expr text;
BEGIN
SELECT INTO _expr
string_agg (op || CASE WHEN attnotnull
THEN quote_ident(attname)
ELSE format('COALESCE(%I, 0)', attname) END
, '')
FROM (
SELECT j->>'name' AS attname
, CASE WHEN j->>'op' = '-' THEN ' - ' ELSE ' + ' END AS op
FROM json_array_elements(_columns) j
) j
JOIN pg_attribute a USING (attname)
WHERE attrelid = _tbl
AND NOT attisdropped
AND attnum > 0;
IF NOT FOUND THEN
RAISE EXCEPTION 'No column found!'; -- or more info
END IF;
RETURN
'SELECT *,' || _expr || ' AS "Account_balance"
FROM ' || _tbl || '
ORDER BY id;';
END
$func$ LANGUAGE plpgsql;
The table itself is parameterized, too. May or may not be useful for you. The only assumption is that every table has an id column for the ORDER BY. Related:
Table name as a PostgreSQL function parameter
I pass columns names and the associated operator as JSON document for flexibility. Only + or - are expected as operator. Input is safely concatenated to make SQL injection impossible.About json_array_elements():
Query for element of array in JSON column
Example call:
SELECT f_build_query('summary', '[{"name":"TransactionA"}
, {"name":"TransactionB", "op": "-"}
, {"name":"TransactionC"}]');
Returns the according valid query string, like:
SELECT *, + COALESCE("TransactionA", 0) - COALESCE("TransactionB", 0) AS "Account_balance"
FROM summary
ORDER BY id;
"TransactionC" isn't there in this case. If both existing columns happen to be NOT NULL, you get instead:
SELECT *, + "TransactionA" - "TransactionB" AS "Account_balance"
FROM summary
ORDER BY id;
db<>fiddle here
You could execute the generated query in the function immediately and return result rows directly. But that's hard as your return type is a combination of a table rows (unknown until execution time?) plus additional column, and SQL demands to know the return type in advance. For just id and sum (stable return type), it would be easy ...
It's odd that your CaMeL-case column names are double-quoted, but the CaMeL-case table name is not. By mistake? See:
Are PostgreSQL column names case-sensitive?
How to pass column names containing single quotes?
Addressing additional question from comment.
If someone used column names containing single quotes by mistake:
CREATE TABLE madness (
id int PRIMARY KEY
, "'TransactionA'" numeric NOT NULL -- you wouldn't do that ...
, "'TransactionC'" numeric NOT NULL
);
For the above function, the JSON value is passed as quoted string literal. If that string is enclosed in single-quotes, escape contained single-quotes by doubling them up. This is required on top of valid JSON format:
SELECT f_build_query('madness', '[{"name":"''TransactionA''"}
, {"name":"TransactionB", "op": "-"}
, {"name":"TransactionC"}]'); --
("''TransactionA''" finds a match, "TransactionC" does not.)
Or use dollar quoting instead:
SELECT f_build_query('madness', $$[{"name":"'TransactionA'"}
, {"name":"TransactionB", "op": "-"}
, {"name":"TransactionC"}]$$);
db<>fiddle here with added examples
See:
Insert text with single quotes in PostgreSQL

Answer 2

Assuming that id is a unique id in summary, then you can use the following trick:
SELECT s.*,
(COALESCE("TransactionA", 0) -
COALESCE("TransactionB", 0) +
COALESCE("TransactionC", 0)
) AS Account_balance
FROM (SELECT id, . . . -- All columns except the TransactionX columns
FROM (SELECT s.*,
(SELECT TransactionA FROM summary s2 WHERE s2.id = s.id) as TransactionA,
(SELECT TransactionB FROM summary s2 WHERE s2.id = s.id) as TransactionB,
(SELECT TransactionC FROM summary s2 WHERE s2.id = s.id) as TransactionC
FROM Summary s
) s CROSS JOIN
(VALUES (NULL, NULL, NULL)) v(TransactionA, TransactionB, TransactionC)
) s
ORDER BY s.id;
The trick here is that the correlated subqueries do not qualify the TransactionA. If the value is defined for summary, then that will be used. If not, it will come from the values() clause in the outer query.
This is a bit of a hack, but it can be handy under certain circumstances.

Answer 3

Check this example:
UPDATE yourtable1
SET yourcolumn = (
CASE
WHEN setting.value IS NOT NULL
THEN CASE WHEN replace(setting.value,'"','') <> '' THEN replace(setting.value,'"','') ELSE NULL END
ELSE NULL
END
)::TIME FROM (SELECT value FROM yourtable2 WHERE key = 'ABC') AS setting;