Is it possible to make a query that changes the where clause acording to some condition? For instance I want to select * from table1 where data is 19/July/2016 but if field id is null then do nothing, else compare id to something else. Like the query bellow?
Select * from table1 where date="2016-07-19" if(isnull(id),"",and id=(select * from ...))
Yes. This should be possible.
If we assume that date and id are references to columns in (the unfortunately named) table table1, if I'm understanding what you are attempting to achieve, we could write a query like this:
SELECT t.id
, t.date
, t....
FROM table1 t
WHERE t.date='2016-07-19'
AND ( t.id IS NULL
OR t.id IN ( SELECT expr FROM ... )
)
It would also be possible to incorporate the MySQL IF() and IFNULL() functions, if there's some requirement to do that.
As far as dynamically changing the text of the SQL statement after the statement is submitted to the database, no, that's not possible. Any dynamic changes to the SQL text would need to be done when the SQL statement is generated, before it is submitted to the database.
My personal preference would be to use a join operation rather than a IN (subquery) predicate.
I think you're trying too hard. If id is NULL that's equivalent to having a FALSE in the where clause. So:
Select * from table1 where date="2016-07-19" and id=(select * from ...)
Should only match the records you want. If id is NULL you get nothing.
Related
I have a table field named category_ids (text) which saves another table's ids as "1,2,3".
Now i want to use this category_ids to a sql IN() query. the query will be like tab1.category_id IN (select category_ids from tab2). but i'm facing issue as select category_ids from tab2 returns '1,2,3' so IN() query not working.
Is there any simple way to convert '1,2,3' to ('1','2','3') or (1,2,3) in sql?
You may use FIND_IN_SET here:
SELECT *
FROM tab1
WHERE FIND_IN_SET(tab1.category_id,
(select category_ids from tab2));
This is just a sample query, your actual one may differ. But the point is that if you want to search for '1' inside a CSV string '1,2,3', then there is a way to do it.
As others have already mentioned, you should avoid storing CSV data in your tables. When I see FIND_IN_SET being heavily used there is usually a smell.
try
tab1.category_id IN (select REPLACE(category_ids, '''', '') from tab2)
If table2 has more than one row, you can use exists with find_in_set():
select t1.*
from table1 t1
where exists (select 1
from table2 t2
where find_in_set(t1.category_id, t2.category_ids) > 0
);
Note that this is a very poor data structure. You should have a single row for each category id in table2. If you did, then the query would be simpler and have better performance.
I was wondering if there's any way to add a subquery with a switch case to the form clause of my select query in order to select a table based on a condition.
For example:
select a.*
from (select (case when (table2.column = 'something')
then (table2.tablename1)
else (table2.tablename2)) as tablename
from table2
where table2.column2 = 'blabla'
limit 1
) a
I tried to write that in many variation & so far non of them worked.
On the most successful tryouts (when I got no mysql errors) it returned the name of the table as the result itself (for example: the value that's in table2.tablename2). I understand why it did that (because I selected everything from a select results...) but how can I use the tablename from the results in order to set the table on the main query?
Hope that make sense...
Any idea?
I cannot create a virtual table for this. Basically what I have, is a list of values:
'Succinylcholine','Thiamine','Trandate','Tridol Drip'
I want to know which of those values is not present in table1 and display them. Is this possible? I have tried using left joins and creating a variable with the list which I can compare to the table, but it returns the wrong results.
This is one of the things I have tried:
SET #list="'Amiodarone','Ammonia Inhalents','Aspirin';
SELECT #list FROM table1 where #list not in (
SELECT Description
FROM table1
);
With only narrow exceptions, you need to have data in table form to be able to obtain those data in your result set. This is the essential problem that all attempts at a solution to this problem run into, given that you cannot create a temporary table. If indeed you can provide the input in any form or format (per your comment), then you can provide it in the form of a subquery:
(
SELECT 'Amiodarone' AS description
UNION ALL
SELECT 'Ammonia Inhalents'
UNION ALL
SELECT 'Aspirin'
)
(Note that that exercises the biggest of the exceptions I noted: you can select scalars directly, without a base table. If you like, you can express that explicitly -- in MySQL and Oracle, at least -- by selecting FROM DUAL.)
In that case, this should work for you:
SELECT
a.description
FROM
(
SELECT 'Amiodarone' AS description
UNION ALL
SELECT 'Ammonia Inhalents'
UNION ALL
SELECT 'Aspirin'
) a
LEFT JOIN table1
ON a.description = table1.description
WHERE table1.description IS NULL
That won't work. the variable's contents will be treated as a monolithic string - one solid block of letters, not 3 separate comma-separated values. The query will be parsed/executed as:
SELECT ... WHERE "'Amio.....rin'" IN (x,y,z,...)
^--------------^--- string
Plus, since you're just doing a sub-select on the very same table, there's no point in this kind of a construct. You could try mysql find_in_set() function:
SELECT #list
FROM table1
WHERE FIND_IN_SET(Description, #list) <> ''
I have a simple query that selects one field and only one row, thus one value.
Is there any way to make it return NULL if the query results in an empty set? Instead of returning zero rows?
I think I need to use something with NOT EXISTS, THEN NULL but not certain about it.
select
(Your entire current Select statement goes here) as Alias
from
dual
dual is a built in table with a single row that can be used for purposes like this. In Oracle this is mandatory. MySQL supports it, but you can also just select a single value without specifying a table, like so:
select
(Your entire current Select statement goes here) as Alias
In either case you're selecting a single value. This means that:
If your select returns one value, that value is returned.
If your select statement returns one column, but no rows, NULL will be returned.
If your select statement returns multiple columns and/or multiple rows, this won't work and the query fails.
An easy way to do this is with aggregation:
select max(col)
from t
where <your condition here>
This always returns one row. If there is no match, it returns NULL.
Late reply but I think this is the easiest method:
SELECT
IFNULL((SELECT your query), NULL)
Use a UNION with a NOT EXISTS(original where clause)
select col1
from mytable
where <some condition>
union
select null
where not exists (
select * from mytable
where <some condition>)
You can use COALESCE for example:
SELECT COALESCE(Field1,NULL) AS Field1 FROM Table1
Edit 1:
sorry i mistake with return field as null not result set,for result set return as null use Union and Exist Function like this:
SELECT NULL AS Field1 FROM Table1 WHERE not EXISTS(SELECT Field1 FROM Table1 WHERE Field2>0)
UNION
SELECT Field1 FROM Table1 WHERE Field2>0
Tables i have
Table1 user_id, date
Table2 user_id, status
I need something like
UPDATE Table2
SET status = 2
WHERE user_id in ( {SELECT user_id FROM Table1 WHERE date > 0} )
In other words i need to see if date in table1 is more than 0000-00-00 then grab user_id of people who match this criteria and use them in table2 to set their status to 2.
Problem is that i need to do it for more than one user so request inside request does not work for me, it only works when there's one row in result.
I think what you've written should work, but what's with the curly braces? Just remove them to make it like this:
UPDATE Table2
SET status = 2
WHERE user_id in (SELECT user_id FROM Table1 WHERE date > 0)
According to the MySQL documentation, IN should work with a subquery.
The ANY keyword, which must follow a comparison operator, means “return TRUE if the comparison is TRUE for ANY of the values in the column that the subquery returns.”
When used with a subquery, the word IN is an alias for = ANY.
Are you using PHP or some other language to make SQL calls? If so, it might easier to just,
firstly: select user ids from table 1
secondly: loop through each row
thirdly: execute the update
Regards,
Mark