I am trying to learn plpgsql code to automate some data cleaning in a database.
My current task is to replace all of '999' values in numeric fields with 'NaN'. What I am trying to do is:
1) find all columns in a schema that are numeric
2) loop through these and use 'update/replace'
My code is below. I think my main problem is finding out how to reference the schema.table in the update statement (but I am sure there are other things I have not done too well).
The error that I am getting is that the relation is not recognised.
Any assistance would be appreciated
Becky
CREATE OR REPLACE FUNCTION household.nodata_replace(schemanm text)
RETURNS VOID as $$
DECLARE
cname text;
tname text;
BEGIN
--FOR col IN
for cname,tname in SELECT column_name::text,table_name::text FROM information_schema.columns
where table_schema = schemanm and data_type in ('integer','double precision')
LOOP
RAISE NOTICE 'cname is: % from %', cname, tname;
EXECUTE 'update '||schemanm::regclass||'.' ||tname::regclass||
' set ' || quote_ident(cname) ||' = replace(' || quote_ident(cname) ||', 999, NaN);';
END LOOP;
END;
$$
LANGUAGE plpgsql;
try to rework your query like following:
EXECUTE 'update '|| (schemanm||'.'||tname)::regclass ||' set ' || quote_ident(cname) ||' = ''NaN'' WHERE '|| quote_ident(cname) ||' = 999;'
because casting to regclass is trying to search within registered relations. and schema is not relation.
or you can
EXECUTE 'update '|| quote_ident(schemanm)||'.'||quote_ident(tname) ||' set ' || quote_ident(cname) ||' = ''NaN'' WHERE '|| quote_ident(cname) ||' = 999;'
I would rather use format() for this. The placeholder %I takes care of properly quoting identifiers if needed.
replace() is for string manipulation not for replacing numbers. To assign the value NaN use set xxx = 'NaN' but you cannot do this for an integer value. Integers do not support NaN
So your dynamic SQL boils down to:
execute format('update %I.%I set %I = ''NaN'' where %I = 999, schemanm, tname, cname, cname);
But you will need to change your where clause to not include integer columns. You probably want to include numeric and real as well:
and data_type in ('numeric','double precision', 'real')
If you just want to mark the "absence of information", I would rather store null in those columns. In that case you don't need to distinguish between the different data types:
execute format('update %I.%I set %I = null where %I = 999, schemanm, tname, cname, cname);
Related
I am trying to write a plpgsql procedure to perform spatial tiling of a postGIS table. I can perform the operation successfully using the following procedure in which the table names are hardcoded. The procedure loops through the tiles in tile_table and for each tile clips the area_table and inserts it into split_table.
CREATE OR REPLACE PROCEDURE splitbytile()
AS $$
DECLARE
tile RECORD;
BEGIN
FOR tile IN
SELECT tid, geom FROM test_tiles ORDER BY tid
LOOP
INSERT INTO split_table (id, areaname, ttid, geom)
SELECT id, areaname, tile.tid,
CASE WHEN st_within(base.geom, tile.geom) THEN st_multi(base.geom)
ELSE st_multi(st_intersection(base.geom, tile.geom)) END as geom
FROM area_table as base
WHERE st_intersects(base.geom, tile.geom);
COMMIT;
END LOOP;
END;
$$ LANGUAGE 'plpgsql';
Having tested this successfully, now I need to convert it to a dynamic procedure where I can provide the table names as parameters. I tried the following partial conversion, using format() for inside of loop:
CREATE OR REPLACE PROCEDURE splitbytile(in_table text, grid_table text, split_table text)
AS $$
DECLARE
tile RECORD;
BEGIN
FOR tile IN
EXECUTE format('SELECT tid, geom FROM %I ORDER BY tid', grid_table)
LOOP
EXECUTE
FORMAT(
'INSERT INTO %1$I (id, areaname, ttid, geom)
SELECT id, areaname, tile.tid,
CASE WHEN st_within(base.geom, tile.geom) THEN st_multi(base.geom)
ELSE st_multi(st_intersection(base.geom, tile.geom)) END as geom
FROM %2$I as base
WHERE st_intersects(base.geom, tile.geom)', split_table, in_table
);
COMMIT;
END LOOP;
END;
$$ LANGUAGE 'plpgsql';
But it throws an error
missing FROM-clause entry for table "tile"
So, how can I convert the procedure to a dynamic one? More specifically, how can I use the record data type (tile) returned by the for loop inside the loop? Note that it works when format is not used.
You can use EXECUTE ... USING to supply parameters to a dynamic query:
EXECUTE
format(
'SELECT r FROM %I WHERE c = $1.val',
table_name
)
INTO result_var
USING record_var;
The first argument to USING will be used for $1, the second for $2 and so on.
See the documentation for details.
Personally I use somehow different way to create dynamic functions. By concatination and execute function. You can also do like this.
CREATE OR REPLACE FUNCTION splitbytile()
RETURNS void AS $$
declare
result1 text;
table_name text := 'test_tiles';
msi text := '+7 9912 231';
msi text := 'Hello world';
code text := 'code_name';
_operator_id integer := 2;
begin
query1 := 'SELECT msisdn from ' || table_name || ' where msisdn = ''' || msi::text ||''';';
query2 := 'INSERT INTO ' || table_name || '(msisdn,usage,body,pr_code,status,sent_date,code_type,operator_id)
VALUES( ''' || msi::text || ''',' || true || ',''' || _body::text || ''',''' || code::text || ''',' || false || ',''' || time_now || ''',' || kod_type || ',' || _operator_id ||');';
execute query1 into result1;
execute query2;
END;
$function$
You just make your query as text then anywhere you want you can execute it. Maybe by checking result1 value inside If statement or smth like that.
I am pretty fresh to PostgreSQL, so please be kind.
I am pretty sure that my problem is that I am mixing plain and dynamic SQL. I have read the relevant documentation but I am not experienced enough to see where I have gone wrong (hoping that my issue is not something more fundamental).
Currently the script is failing with a Query execution error:
SQL Error [42601]: ERROR: syntax error at or near "CREATE"
I intend to use this function to unpivot >9,000 tables (for analysis purposes); fortunately all tables have the same structure.
CREATE OR REPLACE FUNCTION all_schemaTables_unpivot(_schemaName text, _tableName text)
RETURNS void AS
$BODY$
DECLARE
_tbl record;
BEGIN
FOR _tbl IN
SELECT
quote_ident(schemaname) || '.' || quote_ident(tablename) AS fName,
quote_ident(tablename) AS tName
FROM pg_tables
WHERE schemaname = _schemaName
AND tablename LIKE _tableName
LOOP
EXECUTE 'CREATE TABLE ' || _tbl.tName || '_up AS
SELECT region_id, key AS sequential_id, value
FROM (SELECT row_to_json(t.*) AS line, region_id
FROM ' || _tbl.tName || ' AS t) AS r
JOIN LATERAL json_each_text(r.line) ON (key <> "region_id")';
END LOOP;
END;
$BODY$ LANGUAGE plpgsql;
Thanks in advance.
To get the script to work I just needed to fix two things:
I originally did not set appropriate spaces around the query concatenating (thanks #KaushikNayak)
I incorrectly set the quoting values around my dynamic variables (see the 'quote_ident(_tbl.newTableName)' addition. People more expert in PostgreSQL than I will surely have a much cleaner approach - but at least it works!
Moral of the story, sometimes the fix is staring you in the face, but you have been staring at the script for too long! Leave it for awhile and the answer becomes clear.
CREATE OR REPLACE FUNCTION
all_schemaTables_unpivot(_schemaName text, _tableName text)
RETURNS void AS
$BODY$
DECLARE
_tbl record;
BEGIN
FOR _tbl IN
SELECT
quote_ident(schemaname) || '.' || quote_ident(tablename) AS fullNamePath,
quote_ident(tablename) || '_up' AS newTableName
FROM pg_tables
WHERE schemaname = _schemaName
AND tablename LIKE _tableName
LOOP
EXECUTE 'CREATE TABLE '|| quote_ident(_tbl.newTableName) ||' AS
SELECT region_id, key AS sequential_id, value
FROM (SELECT row_to_json(t.*) AS line, region_id
FROM '|| _tbl.fullNamePath ||' AS t) AS r
JOIN LATERAL json_each_text(r.line) ON (key <> "region_id");';
END LOOP;
END;
$BODY$ LANGUAGE plpgsql;
EDIT
It seems my issue is when this select statement returns null (which is the case I'm trying to handle - when it returns null, I want my new value to be -999). How can I go about doing this if it errors out whenever a null is found?
ORIGINAL
I have read every other SO post I could find regarding this error, but none of which seemed to address the root of my issue.
The error is pretty straightforward - one of my arguments within my EXECUTE statement is null. Great. However, I print out each of the values that make up my EXECUTE statement right before it gets called, and I can clearly see that none of the values are null.
Code:
CREATE FUNCTION inform_icrm_prob_flow_query(tablename text, location_id int,
product_date_str text, lead_time_start int,
lead_time_end int, first_member_id int,
last_member_id int, dest_file text)
RETURNS void AS $$
DECLARE
count int;
product_date TIMESTAMPTZ;
interval_lead_time_start text;
interval_lead_time_end text;
curr_value double precision;
query text;
BEGIN
product_date := product_date_str::TIMESTAMPTZ;
count := first_member_id;
curr_value := 0;
interval_lead_time_start := ''''|| product_date ||'''::timestamptz +
interval '''||lead_time_start||' hours''';
interval_lead_time_end := ''''|| product_date ||'''::timestamptz +
interval '''||lead_time_end||' hours'' -
interval ''6 hours''';
--create our temporary table and populate it's date column
EXECUTE 'CREATE TEMPORARY TABLE temp_table_icrm_prob_flow AS
SELECT * FROM generate_series('||interval_lead_time_start || ',' ||
interval_lead_time_end || ', ''6 hours'')
AS date_valid';
LOOP
EXIT WHEN count > last_member_id;
IF NOT EXISTS(
SELECT 'date_valid'
FROM information_schema.columns
WHERE table_name='temp_table_icrm_prob_flow'
and column_name='value'||count||'')
THEN
EXECUTE 'ALTER TABLE temp_table_icrm_prob_flow ADD COLUMN value' || count
|| ' double precision DEFAULT -999';
END IF;
raise notice 'tablename: %', tablename;
raise notice 'location_id: %', location_id;
raise notice 'product_date: %', product_date;
raise notice 'count: %', count;
query := 'SELECT value FROM '|| tablename ||'
INNER JOIN temp_table_icrm_prob_flow
ON (temp_table_icrm_prob_flow.date_valid = '|| tablename ||'.date_valid)
WHERE '|| tablename ||'.id_location = '|| location_id ||'
AND '|| tablename ||'.date_product = '''|| product_date ||'''
AND '|| tablename ||'.id_member = '|| count ||'';
EXECUTE query INTO curr_value;
EXECUTE 'UPDATE temp_table_icrm_prob_flow
SET value'|| count ||' = COALESCE('|| curr_value ||', -999)';
count := count + 1;
END LOOP;
EXECUTE 'ALTER TABLE temp_table_icrm_prob_flow DROP COLUMN date_valid';
EXECUTE 'COPY temp_table_icrm_prob_flow TO '''||dest_file||''' DELIMITER '','' CSV';
EXECUTE 'DROP TABLE temp_table_icrm_prob_flow';
END;
$$ LANGUAGE plpgsql;
Output:
NOTICE: tablename: inform_tseries_data_basin_proc_fcst_prob_flow
NOTICE: location_id: 38
NOTICE: product_date: 2015-02-05 12:00:00+00
NOTICE: count: 1
ERROR: query string argument of EXECUTE is null
CONTEXT: PL/pgSQL function inform_icrm_prob_flow_query(text,integer,text,integer,integer,integer,integer,text) line 38 at EXECUTE
If none of the variables I am passing in are null, and the only other thing referenced is a temp table that I know exists, what could be causing this error?
Note: when changing my query to:
query := 'SELECT value FROM '|| tablename ||' WHERE '|| tablename ||'.id_location = '|| location_id ||' AND '|| tablename ||'.date_product = '''|| product_date ||''' AND '|| tablename ||'.id_member = '|| count ||' AND temp_table_icrm_prob_flow.date_va lid = '|| tablename ||'.date_valid';
I get the following error:
NOTICE: tablename: inform_tseries_data_basin_proc_fcst_prob_flow
NOTICE: location_id: 38
NOTICE: product_date: 2015-02-05 12:00:00+00
NOTICE: count: 1
ERROR: missing FROM-clause entry for table "temp_table_icrm_prob_flow"
LINE 1: ..._data_basin_proc_fcst_prob_flow.id_member = 1 AND temp_table...
^
QUERY: SELECT value FROM inform_tseries_data_basin_proc_fcst_prob_flow WHERE inform_tseries_data_basin_proc_fcst_prob_flow.id_location = 38 AND inform_tseries_data_basin_proc_fcst_prob_flow.date_product = '2015-02-05 12:00:00+00' AND inform_tseries_data_basin_proc_fcst_prob_flow.id_member = 1 AND temp_table_icrm_prob_flow.date_valid = inform_tseries_data_basin_proc_fcst_prob_flow.date_valid
CONTEXT: PL/pgSQL function inform_icrm_prob_flow_query(text,integer,text,integer,integer,integer,integer,text) line 35 at EXECUTE
Sorry for small offtopic. Your code is pretty unreadable (and SQL injecttion vulnerable). There are some techniques, that you can use:
Use clause USING of EXECUTE statement for usual parameters.
DO $$
DECLARE
tablename text := 'mytab';
from_date date := CURRENT_DATE;
BEGIN
EXECUTE 'INSERT INTO ' || quote_ident(tablename) || ' VALUES($1)'
USING from_date;
END
$$;
This code will be safe (due using quote_ident function), little bit faster (due using binary value of from_date variable - removed multiple string<->date conversions and little bit more readable (because string expression is shorter).
Use function format. The building query string will be shorter and more readable (table aliases helps too):
query := format('
SELECT value
FROM %I _dtn
INNER JOIN temp_table_icrm_prob_flow t ON t.date_valid = _dtn.date_valid
WHERE _dtn.id_location = $1
AND _dtn.date_product = $2
AND _dtd.id_member = $3'
, tablename);
EXECUTE query INTO curr_value USING location_id, product_date, count;
Using variables named like important SQL keywords and identifier is wrong idea - names count, values are wrong.
The error message is clean - you are using the identifier temp_table_icrm_prob_flow.date_valid, but the table temp_table_icrm_prob_flow is not mentioned in query. The query missing JOIN part.
My company is going to start generating documents using data from our database and I am designing the function that will spit out the document text. These documents will need to contain data taken from multiple tables, with hundreds of columns and invariably some records will be missing data.
I am trying to make a function that will take null fields and replace them with a little error message that makes it clear to the end user that a piece of data is missing. Since the end user is totally unfamiliar with the backend, I want these messages to reference something intelligible to them.
My solution is pretty simple yet I for the life of me can't get it to work. The record identifier, table name are set as parameters in the function. The function then loops through names for each of the columns in the specified table, building a query that contains a bunch of case statements. Once the loop is complete, the identifier is appended and then the query is executed, returning the results to the calling function.
Despite reading around quite a bit, the best I can is a single column/row containing all the results - not useful to me at all, because I need to be able to easily reference specific pieces of data in the parent query. I am a beginner with Postgres and the documentation is too complex for me to understand, any help would be appreciated.
-- Function: data_handler(text, text)
-- DROP FUNCTION data_handler(text, text);
CREATE OR REPLACE FUNCTION data_handler(target_uri text, target_table TEXT)
RETURNS SETOF record AS
$BODY$
DECLARE
c text;
strSQL text;
site_only text;
result record;
BEGIN
--We need the schema for strSQL but the loop needs just the table name.
site_only = split_part(target_table, '.', 2);
FOR c IN
SELECT column_name
FROM information_schema.columns
WHERE table_name = site_only
LOOP
strSQL = concat(strSQL, chr(10), '(SELECT CASE WHEN ', c::text, '::text IS NULL THEN concat(', chr(39), '<Error:', chr(39), ', (SELECT lkp_value FROM alb_cr.lkp_field_values WHERE column_name = ', chr(39), c::text, chr(39), ')::text, ', chr(39), ' value not found>', chr(39), ')::text ELSE ',
c::text, '::text END AS ', c::text, '_convert) AS ', c::text, ',');
END LOOP;
strSQL = LEFT(strSQL, character_length(strSQL) - 1);
strSQL = concat('SELECT ', strSQL, ' FROM ', target_table, ' WHERE nm_site_id = ', chr(39), target_uri, chr(39));
RETURN QUERY EXECUTE strSQL;
RAISE NOTICE 'strSQL: %', strSQL;
--RETURN strSQL;
--RETURN QUERY EXECUTE format('SELECT ' || strSQL || 'FROM %s WHERE nm_site_id = $1', pg_typeof(target_table)) USING target_uri;
END
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
ALTER FUNCTION data_handler(text, text)
OWNER TO inti;
You could create views for that as well, in the following example on a schema nullsbegone:
-- create the schema to hold the views
create schema if not exists nullsbegone;
-- create a function to create the views (any and all that you might need)
create or replace function nullsbegone.f_make_view_of(p_tablename text) returns void as $f$
begin
execute ($$
create or replace view nullsbegone.$$||(select relname from pg_class where oid = $1::regclass)||$$
returns void as
select $$||array_to_string(array(
select case when not attnotnull then 'COALESCE('||quote_ident(attname)||$$::text, (SELECT '<Error:'''||lkp_value||''' value not found>' FROM alb_cr.lkp_field_values
WHERE column_name = $$||quote_literal(attname)||$$)) AS $$
else '' end || quote_ident(attname)
from pg_attribute
where attrelid = $1::regclass and attnum > 0 order by attnum
), E', \n')||$$
from $$||$1);
end;$f$ language plpgsql;
-- create the view based on a given table
select nullsbegone.f_make_view_of('yourschema.yourtable');
-- select from your view as if you were selecting from the actual table
select * from nullsbegone.yourtable
where nm_site_id = 'yoursite';
Question is simple. How to add column x to table y, but only when x column doesn't exist ? I found only solution here how to check if column exists.
SELECT column_name
FROM information_schema.columns
WHERE table_name='x' and column_name='y';
With Postgres 9.6 this can be done using the option if not exists
ALTER TABLE table_name ADD COLUMN IF NOT EXISTS column_name INTEGER;
Here's a short-and-sweet version using the "DO" statement:
DO $$
BEGIN
BEGIN
ALTER TABLE <table_name> ADD COLUMN <column_name> <column_type>;
EXCEPTION
WHEN duplicate_column THEN RAISE NOTICE 'column <column_name> already exists in <table_name>.';
END;
END;
$$
You can't pass these as parameters, you'll need to do variable substitution in the string on the client side, but this is a self contained query that only emits a message if the column already exists, adds if it doesn't and will continue to fail on other errors (like an invalid data type).
I don't recommend doing ANY of these methods if these are random strings coming from external sources. No matter what method you use (client-side or server-side dynamic strings executed as queries), it would be a recipe for disaster as it opens you to SQL injection attacks.
Postgres 9.6 added ALTER TABLE tbl ADD COLUMN IF NOT EXISTS column_name.
So this is mostly outdated now. You might use it in older versions, or a variation to check for more than just the column name.
CREATE OR REPLACE function f_add_col(_tbl regclass, _col text, _type regtype)
RETURNS bool
LANGUAGE plpgsql AS
$func$
BEGIN
IF EXISTS (SELECT FROM pg_attribute
WHERE attrelid = _tbl
AND attname = _col
AND NOT attisdropped) THEN
RETURN false;
ELSE
EXECUTE format('ALTER TABLE %s ADD COLUMN %I %s', _tbl, _col, _type);
RETURN true;
END IF;
END
$func$;
Call:
SELECT f_add_col('public.kat', 'pfad1', 'int');
Returns true on success, else false (column already exists).
Raises an exception for invalid table or type name.
Why another version?
This could be done with a DO statement, but DO statements cannot return anything. And if it's for repeated use, I would create a function.
I use the object identifier types regclass and regtype for _tbl and _type which a) prevents SQL injection and b) checks validity of both immediately (cheapest possible way). The column name _col has still to be sanitized for EXECUTE with quote_ident(). See:
Table name as a PostgreSQL function parameter
format() requires Postgres 9.1+. For older versions concatenate manually:
EXECUTE 'ALTER TABLE ' || _tbl || ' ADD COLUMN ' || quote_ident(_col) || ' ' || _type;
You can schema-qualify your table name, but you don't have to.
You can double-quote the identifiers in the function call to preserve camel-case and reserved words (but you shouldn't use any of this anyway).
I query pg_catalog instead of the information_schema. Detailed explanation:
How to check if a table exists in a given schema
Blocks containing an EXCEPTION clause are substantially slower.
This is simpler and faster. The manual:
Tip
A block containing an EXCEPTION clause is significantly more
expensive to enter and exit than a block without one.
Therefore, don't use EXCEPTION without need.
Following select query will return true/false, using EXISTS() function.
EXISTS(): The argument of EXISTS is an arbitrary SELECT statement, or
subquery. The subquery is evaluated to determine whether it returns
any rows. If it returns at least one row, the result of EXISTS is
"true"; if the subquery returns no rows, the result of EXISTS is
"false"
SELECT EXISTS(SELECT column_name
FROM information_schema.columns
WHERE table_schema = 'public'
AND table_name = 'x'
AND column_name = 'y');
and use the following dynamic SQL statement to alter your table
DO
$$
BEGIN
IF NOT EXISTS (SELECT column_name
FROM information_schema.columns
WHERE table_schema = 'public'
AND table_name = 'x'
AND column_name = 'y') THEN
ALTER TABLE x ADD COLUMN y int DEFAULT NULL;
ELSE
RAISE NOTICE 'Already exists';
END IF;
END
$$
For those who use Postgre 9.5+(I believe most of you do), there is a quite simple and clean solution
ALTER TABLE if exists <tablename> add if not exists <columnname> <columntype>
the below function will check the column if exist return appropriate message else it will add the column to the table.
create or replace function addcol(schemaname varchar, tablename varchar, colname varchar, coltype varchar)
returns varchar
language 'plpgsql'
as
$$
declare
col_name varchar ;
begin
execute 'select column_name from information_schema.columns where table_schema = ' ||
quote_literal(schemaname)||' and table_name='|| quote_literal(tablename) || ' and column_name= '|| quote_literal(colname)
into col_name ;
raise info ' the val : % ', col_name;
if(col_name is null ) then
col_name := colname;
execute 'alter table ' ||schemaname|| '.'|| tablename || ' add column '|| colname || ' ' || coltype;
else
col_name := colname ||' Already exist';
end if;
return col_name;
end;
$$
This is basically the solution from sola, but just cleaned up a bit. It's different enough that I didn't just want to "improve" his solution (plus, I sort of think that's rude).
Main difference is that it uses the EXECUTE format. Which I think is a bit cleaner, but I believe means that you must be on PostgresSQL 9.1 or newer.
This has been tested on 9.1 and works. Note: It will raise an error if the schema/table_name/or data_type are invalid. That could "fixed", but might be the correct behavior in many cases.
CREATE OR REPLACE FUNCTION add_column(schema_name TEXT, table_name TEXT,
column_name TEXT, data_type TEXT)
RETURNS BOOLEAN
AS
$BODY$
DECLARE
_tmp text;
BEGIN
EXECUTE format('SELECT COLUMN_NAME FROM information_schema.columns WHERE
table_schema=%L
AND table_name=%L
AND column_name=%L', schema_name, table_name, column_name)
INTO _tmp;
IF _tmp IS NOT NULL THEN
RAISE NOTICE 'Column % already exists in %.%', column_name, schema_name, table_name;
RETURN FALSE;
END IF;
EXECUTE format('ALTER TABLE %I.%I ADD COLUMN %I %s;', schema_name, table_name, column_name, data_type);
RAISE NOTICE 'Column % added to %.%', column_name, schema_name, table_name;
RETURN TRUE;
END;
$BODY$
LANGUAGE 'plpgsql';
usage:
select add_column('public', 'foo', 'bar', 'varchar(30)');
Can be added to migration scripts invoke function and drop when done.
create or replace function patch_column() returns void as
$$
begin
if exists (
select * from information_schema.columns
where table_name='my_table'
and column_name='missing_col'
)
then
raise notice 'missing_col already exists';
else
alter table my_table
add column missing_col varchar;
end if;
end;
$$ language plpgsql;
select patch_column();
drop function if exists patch_column();
In my case, for how it was created reason it is a bit difficult for our migration scripts to cut across different schemas.
To work around this we used an exception that just caught and ignored the error. This also had the nice side effect of being a lot easier to look at.
However, be wary that the other solutions have their own advantages that probably outweigh this solution:
DO $$
BEGIN
BEGIN
ALTER TABLE IF EXISTS bobby_tables RENAME COLUMN "dckx" TO "xkcd";
EXCEPTION
WHEN undefined_column THEN RAISE NOTICE 'Column was already renamed';
END;
END $$;
You can do it by following way.
ALTER TABLE tableName drop column if exists columnName;
ALTER TABLE tableName ADD COLUMN columnName character varying(8);
So it will drop the column if it is already exists. And then add the column to particular table.
Simply check if the query returned a column_name.
If not, execute something like this:
ALTER TABLE x ADD COLUMN y int;
Where you put something useful for 'x' and 'y' and of course a suitable datatype where I used int.