I'm curious about how to output 'null' in SQL. In many cases, we need to output 'null' if the targeted value doesn't exist. For example, for this leetcode problem: https://leetcode.com/articles/biggest-single-number/
The correct answer is
select ifnull((select num
from number
group by num
having count(num) = 1
order by num desc
limit 1), null) as num
The other way to write this code is
select ifnull(num, null) as num
from number
group by num
having count(num) = 1
order by num desc
limit 1
However, this code does not output 'null' if the targeted num doesn't exist. I don't understand why and am quite confused.I wonder if anyone can give a detailed explanation over this problem? Thanks a lot!
The IFNULL() function returns the alt_value, if the expression is a NULL
The IFNULL() function returns the expression, if the expression is NOT NULL
https://www.w3schools.com/sql/func_mysql_ifnull.asp
the IFNULL works outside the main query because it is evaluating the result as a whole and not per record.
Related
I have a view xxabc_v (shown below), I need to update the "Code" column to Null wherever it is N/A when "Value" column sum (900+(-900)=0) becomes zero for the "field_name" values (Demand A+Demand B) for the "Date" 01-Apr-21.
How can I put the decode logic to code column in the above case?
Table structure and expected output:
You don't want decode() because a much simpler method works:
select nullif(code, 'N/A')
This returns NULL when code takes on the specified value.
If you actually want to change the data, then you want update:
update t
set code = NULL
where code = 'N/A';
EDIT:
I see, you have an extra condition. So, use case:
(case when code = 'N/A' and
sum(value) over (partition by id, date) = 0
then NULL
else code
end)
I assumed that you need date wise id wise sum when to sum(). Please check this out:
select date,id,(case when sum(value)over(partition by date,id)=0 and code='N/A' then NULL
else Code end)code, field_name,value
from tablename
I'm writing a PostgreSQL function to count the number of times a particular text substring occurs in another piece of text. For example, calling count('foobarbaz', 'ba') should return 2.
I understand that to test whether the substring occurs, I use a condition similar to the below:
WHERE 'foobarbaz' like '%ba%'
However, I need it to return 2 for the number of times 'ba' occurs. How can I proceed?
Thanks in advance for your help.
I would highly suggest checking out this answer I posted to "How do you count the occurrences of an anchored string using PostgreSQL?". The chosen answer was shown to be massively slower than an adapted version of regexp_replace(). The overhead of creating the rows, and the running the aggregate is just simply too high.
The fastest way to do this is as follows...
SELECT
(length(str) - length(replace(str, replacestr, '')) )::int
/ length(replacestr)
FROM ( VALUES
('foobarbaz', 'ba')
) AS t(str, replacestr);
Here we
Take the length of the string, L1
Subtract from L1 the length of the string with all of the replacements removed L2 to get L3 the difference in string length.
Divide L3 by the length of the replacement to get the occurrences
For comparison that's about five times faster than the method of using regexp_matches() which looks like this.
SELECT count(*)
FROM ( VALUES
('foobarbaz', 'ba')
) AS t(str, replacestr)
CROSS JOIN LATERAL regexp_matches(str, replacestr, 'g');
How about use a regular expression:
SELECT count(*)
FROM regexp_matches('foobarbaz', 'ba', 'g');
The 'g' flag repeats multiple matches on a string (not just the first).
There is a
str_count( src, occurence )
function based on
SELECT (length( str ) - length(replace( str, occurrence, '' ))) / length( occurence )
and a
str_countm( src, regexp )
based on the #MikeT-mentioned
SELECT count(*) FROM regexp_matches( str, regexp, 'g')
available here: postgres-utils
Try with:
SELECT array_length (string_to_array ('1524215121518546516323203210856879', '1'), 1) - 1
--RESULT: 7
Why does
select *
from (
SELECT LEVEL as VAL
FROM DUAL
CONNECT BY LEVEL <= 1000
ORDER BY LEVEL
) n
left outer join (select to_number(trim(alphanumeric_column)) as nr from my_table
where NOT regexp_like (trim(alphanumeric_column),'[^[:digit:]]')) d
on n.VAL = d.nr
where d.nr is null
and n.VAL >= 100
throw a ORA-01722 invalid number (reason is the last row, n.VAL), whereas the similar version with numeric columns im my_table works fine:
select *
from (
SELECT LEVEL as VAL
FROM DUAL
CONNECT BY LEVEL <= 1000
ORDER BY LEVEL
) n
left outer join (select numeric_column as nr from my_table) d
on n.VAL = d.nr
where d.nr is null
and n.VAL >= 100
given that numeric_column is of type number and alphanumeric_column of type nvarchar_2. Note that the upper example works fine without the numerical comparison (n.VAL >= 100).
Does anybody know?
This problem was driving me crazy. I narrowed the problem to a simpler query
SELECT *
FROM (SELECT TO_NUMBER(TRIM (alphanumeric_column)) AS nr
FROM my_table
WHERE NOT REGEXP_LIKE (TRIM (alphanumeric_column), '[^[:digit:]]')) d
WHERE d.nr > 1
With alphanumeric_colum values of ('100','200','XXXX'); Running the above statement gave the "invalid number" error. I then made a slight change to the query to use the CAST function instead of TO_NUMBER:
SELECT *
FROM (SELECT CAST (TRIM (alphanumeric_column) AS NUMBER) AS nr
FROM my_table
WHERE NOT REGEXP_LIKE (TRIM (alphanumeric_column), '[^[:digit:]]')) d
WHERE d.nr > 1
And this correctly returned - 100, 200. I would think that those functions would be similar in behavior. It almost appears as though oracle is trying to evaluate the d.nr > 1 constraint before the view is constructed, which makes no sense. If anyone can shed light on why this is happening, I would be grateful. See SQLFiddle example
UPDATE: I did some more digging, because I don't like not knowing why something just works. I ran EXPLAIN PLAN on both queries and got some interesting results.
For the query that failed, the predicate information looks like this:
1 - filter(TO_NUMBER(TRIM("ALPHANUMERIC_COLUMN"))>1 AND NOT
REGEXP_LIKE (TRIM("ALPHANUMERIC_COLUMN"),'[^[:digit:]]'))
You will notice that the TO_NUMBER function is called first in the AND condition, then
the regexp to exclude alpha values. I am thinking oracle maybe does a short-circuit evaluation with the AND condition, and since it is executing TO_NUMBER first, it fails.
However, when we use the CAST function, the evaluation order is swapped, and the
regexp exclusion is evaluated first. Since for the alpha values, it is false, then the
second part of the AND clause is not evaluated, and the query works.
1 - filter( NOT REGEXP_LIKE (TRIM("ALPHANUMERIC_COLUMN"),'[^[:digit:]
]') AND CAST(TRIM("ALPHANUMERIC_COLUMN") AS NUMBER)>1)
Oracle can be strange sometimes.
I believe when it comes to the Predicate (where) clause, Oracle can/will reorder the entire plan as it sees fit. So with regard to the predicate, it will short-circuit (as OldProgrammer noted) the evaluation however it wants, and you wont be able to guarantee the exact order it occurs.
In your current SQL, you are depending on the predicate to remove non numbers. One option would be to not use "WHERE NOT regexp_like ..." and instead use regexp_substr with coalesce. For example:
create table t_tab2
(
col varchar2(10)
);
create index t_tab2_idx on t_tab2(col);
insert into t_tab2
select level from dual
connect by level <= 100;
insert into t_tab2 values ('123ABC456');
commit;
-- select values > 95 (96->100 exclude non numbers)
select d.* from
(
select COALESCE(TO_NUMBER(REGEXP_SUBSTR(trim(col), '^\d+$')), 0) as nr
from t_tab2
) d
where d.nr > 95;
This should run without throwing invalid number error. Note that the coalesce will return the number 0 for any non numbers coming from the data, you may want to change that based on your needs and data.
I am having trouble with some sql. When I run the following query:
Select * from teleapp;
I get TONS of results. Resulst which include a column (called cashwithappyn) that has TONS of empty or null data cells. (They look empty and don't say null)
The column info is:
ColumnName ID Null? Data Type Histogram Num Distinct Num Nulls Density
CASHWITHAPPYN 54 Y VARCHAR2(1 Byte) Frequency 2 56895 0
When I try to run the following query:
Select * from teleapp where cashwithappyn = null;
or
Select * from teleapp where cashwithappyn = '';
or
Select * from teleapp where cashwithappyn not like '';
or
Select * from teleapp where cashwithappyn not in ('Y','N');
or ANY type of combination, I cannot seem to get all of the rows with nothing in cashwithappyn.
Any ideas? Please help, this is the last part of a project that I was assigned to do and I just need to figure this out.
Thanks.
Maybe the column contains blanks. In that case you can do
WHERE TRIM(CASHWITHAPYYN) IS NULL
TRIM removes all blanks before and after and if nothing is left anymore the value becomes NULL
e.g.
TRIM(' ') IS NULL -- one blank removed = true
TRIM(NULL) IS NULL -- true
Also NULL cannot be compared with = NULL but must be phrased IS NULL. NULL is not a value as such and that is why the comparison never works.
You need to use IS NULL
Select * from teleapp where cashwithappyn is null;
Logical test expressions (=, Not In, Like etc) with null result in a false so all of the following result in false
1 = NULL
1 <> NULL
NULL = NULL
NULL <> NULL
NULL NOT IN ('a','b')
NULL Not Like NULL
Additionally in oracle the zero length string is null so NOT LIKE '' will never return any rows
You'll need to use either is null, is not null or Coalesce
A co-worker and I did a bunch of research, here's what we came up with that will work. Any ideas why this works but not the others?
Select * from teleapp where nvl(cashwithappyn,'U') = 'U';
I have seen similar questions asked elsewhere on this site, but more in the context of optimization.
I am having an issue with the order of execution of the conditions in a WHERE clause. I have a field which stores codes, most of which are numeric but some of which contain non-numeric characters. I need to do some operations on the numeric codes which will cause errors if attempted on non-numeric strings. I am trying to do something like
WHERE isnumeric(code) = 1
AND CAST(code AS integer) % 2 = 1
Is there any way to make sure that the isnumeric() executes first? If it doesn't, I get an error...
Thanks in advance!
The only place order of evaluation is guaranteed is CASE
WHERE
CASE WHEN isnumeric(code) = 1
THEN CAST(code AS integer) % 2
END = 1
Also just because it passes the isnumeric test doesn't guarantee that it will successfully cast to an integer.
SELECT ISNUMERIC('$') /*Returns 1*/
SELECT CAST('$' AS INTEGER) /*Fails*/
Depending upon your needs you may find these alternatives preferable.
Why not simply do it using LIKE?:
Where Code Not Like '%[^0-9]%'
Btw, either using my solution or using IsNumeric, there are some edge cases which might lead one to using a UDF such as 1,234,567 where IsNumeric will return 1 but Cast will throw an exception.
Why not use a CASE statement to say something like:
WHERE
CASE WHEN isnumeric(code) = 1
THEN CAST(code AS int) % 2 = 1
ELSE /* What ever else if not numeric */ END
You could do it in a case statement in the select clause, then limit by the value in an outer select
select * from (
select
case when isNum = 1 then CAST(code AS integer) % 2 else 0 end as castVal
from (
select
Case when isnumeric(code) = 1 then 1 else 0 end as isNum
from table) t
) t2
where castval = 1