SQL. Filter null fields in resonse - mysql

Question

I have table users, with fields: id, name, age, gender, country, city, comment. Fields maybe null. For example:
cursor.execute('select * from users where id = 12')
cursor.fetchone()
(12, 'alex', 33, 'male', None, None, None)
How I can get back from query only not null fields?
This query must return me just
(12, 'alex', 33, 'male')
I can easily do it with a programming language, but I losing my resources by getting redundant info from tables and it also forced me to add redundant code.

Answer 1

So, you need to get customized table-oriented result with columns dynamically generated with SQL ANSI.
You will need a kit with two procedures:
Check if a column has only NULL values or not.
Gather all columns with NO NULL values.
With these two procedures you can check each column of a table strcuture and get an output withtout columns with NULL values.
The first procedure is generic:
set delimiter //
create procedure check_field_null(col varchar(64),
schemaname varchar(255),
tablename varchar(255),
out QN int)
BEGIN
SET #sSQL = concat('SELECT #N := COUNT(*) FROM ', schemaname, '.', tablename , ' WHERE (', col, ' <=> NULL);');
prepare stm from #sSQL;
execute stm;
set QN =#N;
deallocate prepare stm;
END
//
The second procedure means that you have the above mentioned method to identify NOT NULLS columns and put all to search.
set delimiter //
create procedure cur_cs_customer(inout allcols varchar(255), in my_schema_name varchar(64), in my_table_name varchar(64))
BEGIN
DECLARE Count_Null int default 0;
DECLARE initial INT DEFAULT 0;
DECLARE MYCOL char(64);
DECLARE ch_done INT DEFAULT 0;
DECLARE cs_cur1 CURSOR
FOR SELECT C.COLUMN_NAME
FROM information_schema.COLUMNS C
WHERE C.TABLE_SCHEMA = my_schema_name
AND C.TABLE_NAME = my_table_name;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET ch_done = true;
open cs_cur1;
read_cs_cur1:
LOOP
FETCH cs_cur1 INTO MYCOL;
IF (ch_done ) THEN
LEAVE read_cs_cur1;
END IF;
IF NOT isnull(MYCOL) THEN
call check_field_null(MYCOL,
my_schema_name,
my_table_name,
Count_Null);
if Count_Null = 0 then
/* Only it inlcudes fields with not null values */
set initial = initial + 1;
if initial = 1 then
SET allcols = MYCOL;
else
SET allcols = concat( cast(allcols as char(255)), ',', MYCOL);
end if;
end if;
END IF;
END LOOP read_cs_cur1;
close cs_cur1;
select allcols;
END
//
After, you may run as follows:
set delimiter ;
call cur_cs_customer(#my_args, 'schema_name', 'users');
select #my_args;
set #stm = concat('SELECT ', #my_args, ' FROM users;');
PREPARE stmt1 FROM #stm;
execute stmt1;
deallocate prepare stmt1;
Further explanation may be checked in an article here! .
I hope this tip may help you.