Suppose a table has a column num of type INT and the values in that column are allowed to be NULL. Now, provided that some rows has num cell set to some value and it's NULL for other rows, how do I select all the rows where num is not equal to a specific value, including the rows where num is NULL, using just one condition?
For example, if the num value I wanted to exclude from selection was 5, I would have to use a SELECT query with two conditions:
SELECT * FROM `table` WHERE `num` != 5 OR `num` IS NULL;
But how to make this simple retrieval using just one condition?
hope this will help you.
SELECT * FROM `table` WHERE ifnull(`num`,0) != 5;
Related
I would like to see if in my table there exists a column where the entirety of its rows are null.
SELECT * FROM yourTableName
WHERE yourSpecificColumnName IS NULL
-> this will only return the values that are null but i wont know if yourSpecificColumnName is entirely null throughout the table
Using COUNT combined with HAVING:
COUNT
Returns either the number of non-NULL records for the specified columns, or the total number of records.
SELECT 'Entire_column_is_empty'
FROM yourTable
HAVING COUNT(yourSpecificColumnName) = 0;
or QUALIFY:
SELECT *
FROM yourTable
QUALIFY COUNT(yourSpecificColumnName) OVER() = 0;
Alternative approach:
SELECT 'Entire_column_is_empty'
FROM yourTable
HAVING MIN(yourSpecificColumnName) = MAX(yourSpecificColumnName);
Scenario: I have a table with duplicate data. One of the columns of this table is ddate, if it is empty/null I want to select that row (and remove it). But for some reason, I cannot find the null rows with a direct query.
Issue: When I run the following query (1):
select
`ddate`,
count(1) as `nb`
from instrument_nt
group by `ddate`;
I get the number of rows where ddate is NULL and where it has other values. But when I run query (2):
select count(*) from instrument_nt where `ddate` = Null;
or
select * from instrument_nt where `ddate` = NULL;
My query result is either 0 or empty.
Question: What is the difference between those two queries (1 and 2)? How can I properly delete the data that has null/missing dates?
NULL mean unknow it's a value.
If you want to get NULL row you need to use IS NULL instead of eqaul NULL
select count(*) from instrument_nt where `ddate` IS Null;
What is the difference between those two queries (1 and 2)? How can I properly delete the data that has null/missing dates?
(1)
select count(*) from instrument_nt where `ddate` IS Null;
you will get the amount ddate is NULL from instrument_nt table.
(2)
select * from instrument_nt where `ddate` IS NULL;
you will get a result set which ddate is NULL;
Every null is defined to be different from every other null. Thus, equality to null is always false. See, for example, here, which describes this so-called "three value problem".
For this third class of value, you want to use IS, as in IS NULL or IS NOT NULL.
use the keyword IS NULL to check the null values in tables
For example:
select * from instrument_nt where `ddate` IS NULL;
MySQL null checks use the IS operator instead of =.
Your query should look like this: select * from instrument_nt whereddateIS NULL;
select * from items where id is not null;
This query returns all correct records, but this one :
select * from items where id not in (null);
shows nothing. Can somebody explain why this happens?
null is not value, that is why we use is not null or is null expression.
in your second case
select * from items where id not in (null);
is equal to
select * from items where id != null;
which evaluates nothing because id is neither satisfies = null nor satisfies != null
Your second query will not work because null is not a value, it is undefined, that's why you can't use it with any sql operator. Instead you can re-write your second query like this :
select * from items where coalesce(id, '') not in ('');
But this will also exclude the result for id = '' So use of the first query is the best solution in this case which is :
select * from items where id is not null;
Get more information about NULL in mysql from Here
In a give table I want to select the row with the maximum value in column-A, but in the same time if there are 2 or more rows that have a maximum value a want to get the row where the value in column-B is not null, if there is one, otherwise I just get the first row with the maximum value even if the value in Column B is null.
In summary:
first choice : Column-A maximum, Column-B Not null
in case such a row does not exist
second choice : Column-A maxim, Column-B Null
Is it possible to write a single query with this constrain or do I have to create 2 queries with some logic in between?
You can use multiple columns with ORDER BY like below
SELECT columnA,columnB
FROM YourTable
ORDER BY columnA DESC, columnB DESC
It'll order by columnA first descending order, then columnB in descending order the null will be the last choice.
add a LIMIT 1 at end of query if you just one row returned.
Below Query will do the trick
SELECT * FROM tableA where columnA=(select max(columnA) from tableA) order by columnB desc
You could use an inline view to return the maximum value of col_a, and then do a join to get all the rows that have the same col_a value, then get the maximum value of col_b from those rows.
For example:
SELECT s.col_a
, MAX(t.col_b) AS col_b
FROM ( SELECT MAX(r.col_a) AS col_a
FROM mytable r
) s
JOIN mytable t
ON t.col_a = s.col_a
I have this kind of simple query that returns a not null integer field for a given id:
SELECT field1 FROM table WHERE id = 123 LIMIT 1;
The thing is if the id is not found, the resultset is empty. I need the query to always return a value, even if there is no result.
I have this thing working but I don't like it because it runs 2 times the same subquery:
SELECT IF(EXISTS(SELECT 1 FROM table WHERE id = 123) = 1, (SELECT field1 FROM table WHERE id = 123 LIMIT 1), 0);
It returns either field1 if the row exists, otherwise 0. Any way to improve that?
Thanks!
Edit following some comments and answers: yes it has to be in a single query statement and I can not use the count trick because I need to return only 1 value (FYI I run the query with the Java/Spring method SimpleJdbcTemplate.queryForLong()).
MySQL has a function to return a value if the result is null. You can use it on a whole query:
SELECT IFNULL( (SELECT field1 FROM table WHERE id = 123 LIMIT 1) ,'not found');
As you are looking for 1 record, (LIMIT 1) then this will work.
(SELECT field1 FROM table WHERE id = 123)
UNION
(SELECT 'default_value_if_no_record')
LIMIT 1;
Can be a handy way to display default values, or indicate no results found. I use it for reports.
See also http://blogs.uoregon.edu/developments/2011/03/31/add-a-header-row-to-mysql-query-results/ for a way to use this to create headers in reports.
You could include count(id). That will always return.
select count(field1), field1 from table where id = 123 limit 1;
http://sqlfiddle.com/#!2/64c76/4
You can use COALESCE
SELECT COALESCE(SUM(column),0)
FROM table
If someone is looking to use this to insert the result INTO a variable and then using it in a Stored Procedure; you can do it like this:
DECLARE date_created INT DEFAULT 1;
SELECT IFNULL((SELECT date FROM monthly_comission WHERE date = date_new_month LIMIT 1), 0)
INTO date_created
WHERE IFNULL((SELECT date FROM monthly_comission WHERE date = date_new_month LIMIT 1), 0) = 0;
With this you're storing in the variable 'date_created' 1 or 0 (if returned nothing).
Do search with LEFT OUTER JOIN. I don't know if MySQL allows inline VALUES in join clauses but you can have predefined table for this purposes.
k-a-f's answer works for selecting one column, if selecting multiple column, we can.
DECLARE a BIGINT DEFAULT 1;
DECLARE b BIGINT DEFAULT "name";
SELECT id, name from table into a,b;
Then we just need to check a,b for values.
if you want both always a return value but never a null value you can combine count with coalesce :
select count(field1), coalesce(field1,'any_other_default_value') from table;
that because count, will force mysql to always return a value (0 if there is no values to count) and coalesce will force mysql to always put a value that is not null