Postgresql – use variable to determine the name of a column in the select in postgre

dynamic-sqlpostgresqlscriptingselect

I'm trying create a update for a new structure of tables from a old table, but not using functions. I'm trying create a script to it.
The old table is like this:

-- old table (OldTable)
name| col_a | col_b | col_c |
--------|---------|---------|--------|
ABC  |     0    |NULL |    1    |
DEF  |     1    |    1     |    1   |
GHI   | NULL |    1    |     0   |

And, new tables:
-- Table Users

ID | NAME
----| ---------
 1  | ABC
 2  | DEF
 3  | GHI

-- Table Rules
ID | RULE_NAME
--- | ------------------
 1  | col_a
 2  | col_b
 3  | col_c
 4  | col_d

-- Table UserRules
ID_USER | ID_RULE
------------- |-------------

So, I'm needing populate the table UserRules with the result from a select on TableMix where the user name is equals to the user name on table User and the value for column in TableMix is equals 1. (select from OldTable where OldTable.name = Users.name).
Well, I'm trying this:

DO $$
    DECLARE rules CURSOR FOR SELECT column_name FROM information_schema.columns
        WHERE table_schema = 'public' AND table_name   = 'OldTable' 
        AND data_type = 'numeric' AND column_name NOT IN ('foo','bar'); 
        -- "foo" and "bar" are another numerc cols, but not define rules.
    DECLARE users CURSOR FOR SELECT name FROM public.Users;
    BEGIN
        FOR ruleName IN rules LOOP
            FOR userName IN users LOOP
                EXECUTE format('SELECT COALESCE(%I,col,$1) FROM public.OldTable 
                    WHERE name = ''$2''', ruleName, username);      
                -- insert on... populate the table Rules after get the id of the rule in table Rules if the result of select in OldTable equals 1 (or true if has a "where ... = 1")
            END LOOP;
        END LOOP;
END $$

After this, I don't have progress…
Someone help me? Thans!

UPDATE

This way I can get the real value:

DO $$
DECLARE 
    rules CURSOR FOR SELECT column_name FROM information_schema.columns
    WHERE table_schema = 'public' AND table_name   = 'oldtable' AND data_type = 'numeric'
    AND column_name NOT IN ('foo','bar');

    names CURSOR FOR SELECT username FROM public.users;

    res integer;
BEGIN
    FOR rulename IN rules LOOP          
        EXECUTE format('SELECT %s FROM oldtable WHERE nome = %L', rulename, 'USER NAME')    INTO res;
    -- IF res > 0 THEN
       RAISE NOTICE '%', res;
    --END IF;
    END LOOP;
END $$

Result the real integer value.

But if I use a loop for usernames, it's doesn't work.

DO $$
DECLARE 
    rules CURSOR FOR SELECT column_name FROM information_schema.columns
    WHERE table_schema = 'public' AND table_name   = 'oldtable' AND data_type = 'numeric'
    AND column_name NOT IN ('foo','bar');

    names CURSOR FOR SELECT username FROM public.users;

    res integer;
BEGIN
    FOR name IN names LOOP
        FOR rulename IN rules LOOP          
            EXECUTE format('SELECT %s FROM oldtable WHERE nome = %L', rulename, name)   INTO res;
        -- IF res > 0 THEN
           RAISE NOTICE '%', res;
        --END IF;
        END LOOP;
    END LOOP;
END $$

So, the resul is always .
Where am I going wrong?

Best Answer

I think you might be looking for the USING clause:

EXECUTE format('SELECT COALESCE(%I,col,$1) FROM public.TableMix 
                WHERE name = $2', ruleName)
        USING (username, whateverTheSecondParameterIs);

You use $1, $2, etc for placement-parameters, passed by USING. You use %I for itentifiers expanded by format.

Update: If you want to coalesce the column name ruleName if it's null, perhaps you want something more like:

EXECUTE format('SELECT %I FROM public.TableMix 
                WHERE name = $1', coalesce(ruleName, 'col'))
        USING (username);

??

You need to think about order of evaluation. What's part of the generated SQL, and what's evaluated in order to create that SQL?

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