Solution to "subquery returns more than 1 row" error

Question

I have one query that returns multiple rows, and another query in which I want to set criteria to be either one of values from those multiple rows , so basicly I want the subquery to look something like this:

select * 
from table
where id= (multiple row query);

Where multiple row query returns multiple rows. So if the values from those rows are 1,2,3 then I want to set id to be 1 or 2 or 3.

Answer 1

= can be used when the subquery returns only 1 value.

When subquery returns more than 1 value, you will have to use IN:

select * 
from table
where id IN (multiple row query);

For example:

SELECT *
FROM Students
WHERE Marks = (SELECT MAX(Marks) FROM Students)   --Subquery returns only 1 value

SELECT *
FROM Students
WHERE Marks IN 
      (SELECT Marks 
       FROM Students 
       ORDER BY Marks DESC
       LIMIT 10)                       --Subquery returns 10 values

Answer 2

You can use in():

select * 
from table
where id in (multiple row query)

or use a join:

select distinct t.* 
from source_of_id_table s
join table t on t.id = s.t_id
where <conditions for source_of_id_table>

The join is never a worse choice for performance, and depending on the exact situation and the database you're using, can give much better performance.

Answer 3

Use MAX in your SELECT query to return one value… EXAMPLE

INSERT INTO school_year_studentid (student_id,syr_id) VALUES
((SELECT MAX(student_id) FROM student), (SELECT MAX(syr_id) FROM school_year))

instead of

INSERT INTO school_year_studentid (student_id,syr_id) VALUES
((SELECT (student_id) FROM student), (SELECT (syr_id) FROM school_year))

try it without MAX it will return more than one value