Id like to run a query only if a value in the last row is correct. In my exemple if the value in ColumnA is 1 on the last row then i want to run MyQuery. But if the value is not 1 stop there and do nothing.
i've try with case and count(*) and also with If exists. but i keep getting error
SELECT CASE WHEN ((SELECT COUNT(*) FROM
(Select a.* from table as a order by a.index desc limit 1) as b
where b.ColumnA = 1)) > 0 )
THEN (MyQuery)
END
i've also try with if exists but it doesn'work either
if exists Select b.* from (Select a.* from table as a order by a.index desc limit 1) where b.ColumnA = 1
begin
(MyQuery)
end
can you point me what wrong in those query or maybee there's a better way to achive this.
EDIT. This query will be run on a trigger after each insert in that table the goal is to avoid running MyQuery on row that dont required it. MyQuery is a bit slow and most row dont required it to run.
I think we can rephrase your logic here to make it work as you want:
WITH cte AS (
SELECT ColumnA, ROW_NUMBER() OVER (ORDER BY index DESC) rn
FROM yourTable
)
(your query here)
WHERE (SELECT ColumnA FROM cte WHERE rn = 1) = 1;
The WHERE clause above would return either true or false, and would apply to all records in the potential result set from your query. That is, if the ColumnA value from the "last" record were 1, then you would get back the entire result set, otherwise it would be empty set.
Assuming your version of MariaDB supports neither ROW_NUMBER nor CTEs, then use:
(your query here)
WHERE (SELECT ColumnA FROM yourTable ORDER BY index DESC LIMIT 1) = 1;
It depends on what your query is.
INSERT ...
SELECT ... WHERE ... -- this could lead to zero rows being inserted
DELETE ...
WHERE NOT EXISTS ( SELECT ... ) -- this could lead to zero rows being deleted
UPDATE t1 JOIN t2 ... -- the JOIN may cause no rows to be updated
Note:
(Select a.* from table as a order by a.index desc limit 1) as b
where b.ColumnA = 1)) > 0 )
can be simplified (and sped up) to
( ( SELECT ColumnA FROM table ORDER BY index DESC LIMIT 1 ) = 1 )
Note that that is a true/false "expression", so it can be used in various places.
I want to use ORDER BY on every UNION ALL queries, but I can't figure out the right syntax. This is what I want:
(
SELECT id, user_id, other_id, name
FROM tablename
WHERE user_id = 123 AND user_in IN (...)
ORDER BY name
)
UNION ALL
(
SELECT id, user_id, other_id, name
FROM tablename
WHERE user_id = 456 AND user_id NOT IN (...)
ORDER BY name
)
EDIT:
Just to be clear: I need two ordered lists like this, not one:
1
2
3
1
2
3
4
5
Thank you very much!
Something like this should work in MySQL:
SELECT a.*
FROM (
SELECT ... FROM ... ORDER BY ...
) a
UNION ALL
SELECT b.*
FROM (
SELECT ... FROM ... ORDER BY ...
) b
to return rows in an order we'd like them returned. i.e. MySQL seems to honor the ORDER BY clauses inside the inline views.
But, without an ORDER BY clause on the outermost query, the order that the rows are returned is not guaranteed.
If we need the rows returned in a particular sequence, we can include an ORDER BY on the outermost query. In a lot of use cases, we can just use an ORDER BY on the outermost query to satisfy the results.
But when we have a use case where we need all the rows from the first query returned before all the rows from the second query, one option is to include an extra discriminator column in each of the queries. For example, add ,'a' AS src in the first query, ,'b' AS src to the second query.
Then the outermost query could include ORDER BY src, name, to guarantee the sequence of the results.
FOLLOWUP
In your original query, the ORDER BY in your queries is discarded by the optimizer; since there is no ORDER BY applied to the outer query, MySQL is free to return the rows in whatever order it wants.
The "trick" in query in my answer (above) is dependent on behavior that may be specific to some versions of MySQL.
Test case:
populate tables
CREATE TABLE foo2 (id INT PRIMARY KEY, role VARCHAR(20)) ENGINE=InnoDB;
CREATE TABLE foo3 (id INT PRIMARY KEY, role VARCHAR(20)) ENGINE=InnoDB;
INSERT INTO foo2 (id, role) VALUES
(1,'sam'),(2,'frodo'),(3,'aragorn'),(4,'pippin'),(5,'gandalf');
INSERT INTO foo3 (id, role) VALUES
(1,'gimli'),(2,'boromir'),(3,'elron'),(4,'merry'),(5,'legolas');
query
SELECT a.*
FROM ( SELECT s.id, s.role
FROM foo2 s
ORDER BY s.role
) a
UNION ALL
SELECT b.*
FROM ( SELECT t.id, t.role
FROM foo3 t
ORDER BY t.role
) b
resultset returned
id role
------ ---------
3 aragorn
2 frodo
5 gandalf
4 pippin
1 sam
2 boromir
3 elron
1 gimli
5 legolas
4 merry
The rows from foo2 are returned "in order", followed by the rows from foo3, again, "in order".
Note (again) that this behavior is NOT guaranteed. (The behavior we observer is a side effect of how MySQL processes inline views (derived tables). This behavior may be different in versions after 5.5.)
If you need the rows returned in a particular order, then specify an ORDER BY clause for the outermost query. And that ordering will apply to the entire resultset.
As I mentioned earlier, if I needed the rows from the first query first, followed by the second query, I would include a "discriminator" column in each query, and then include the "discriminator" column in the ORDER BY clause. I would also do away with the inline views, and do something like this:
SELECT s.id, s.role, 's' AS src
FROM foo2 s
UNION ALL
SELECT t.id, t.role, 't' AS src
FROM foo3 t
ORDER BY src, role
Don't use ORDER BY in an individual SELECT statement inside a UNION, unless you're using LIMIT with it.
The MySQL docs on UNION explain why (emphasis mine):
To apply ORDER BY or LIMIT to an individual SELECT, place the clause
inside the parentheses that enclose the SELECT:
(SELECT a FROM t1 WHERE a=10 AND B=1 ORDER BY a LIMIT 10) UNION
(SELECT a FROM t2 WHERE a=11 AND B=2 ORDER BY a LIMIT 10);
However, use of ORDER BY for individual SELECT statements implies
nothing about the order in which the rows appear in the final result
because UNION by default produces an unordered set of rows. Therefore,
the use of ORDER BY in this context is typically in conjunction with
LIMIT, so that it is used to determine the subset of the selected rows
to retrieve for the SELECT, even though it does not necessarily affect
the order of those rows in the final UNION result. If ORDER BY appears
without LIMIT in a SELECT, it is optimized away because it will have
no effect anyway.
To use an ORDER BY or LIMIT clause to sort or limit the entire UNION
result, parenthesize the individual SELECT statements and place the
ORDER BY or LIMIT after the last one. The following example uses both
clauses:
(SELECT a FROM t1 WHERE a=10 AND B=1)
UNION
(SELECT a FROM t2 WHERE a=11 AND B=2)
ORDER BY a LIMIT 10;
It seems like an ORDER BY clause like the following will get you what you want:
ORDER BY user_id, name
You just use one ORDER BY at the very end.
The Union turns two selects into one logical select. The order-by applies to the entire set, not to each part.
Don't use any parens either. Just:
SELECT 1 as Origin, blah blah FROM foo WHERE x
UNION ALL
SELECT 2 as Origin, blah blah FROM foo WHERE y
ORDER BY Origin, z
(SELECT id, user_id, other_id, name
FROM tablename
WHERE user_id = 123
AND user_in IN (...))
UNION ALL
(SELECT id, user_id, other_id, name
FROM tablename
WHERE user_id = 456
AND user_id NOT IN (...)))
ORDER BY name
You can also simplify this query:
SELECT id, user_id, other_id, name
FROM tablename
WHERE (user_id = 123 AND user_in IN (...))
OR (user_id = 456 AND user_id NOT IN (...))
Lets say I have a MySQL table that has the following entries:
1
2
3
2
5
6
7
6
6
8
When I do an "SELECT * ..." I get back all the entries. But I want to get back only these entries, that exist only once within the table. Means the rows with the values 2 (exists two times) and 6 (exists three times) have to be dropped completely out of my result.
I found a keyword DISTINCT but as far as I understood it only avoids entries are shown twice, it does not filters them completely.
I think it can be done somehow with COUNT, but all I tried was not really successful. So what is the correct SQL statement here?
Edit: to clarify that, the result I want to get back is
1
3
5
7
8
You can use COUNT() in combination with a GROUP BY and a HAVING clause like this:
SELECT yourCol
FROM yourTable
GROUP BY yourCol
HAVING COUNT(*) < 2
Example fiddle.
You want to mix GROUP BY and COUNT().
Assuming the column is called 'id' and the table is called 'table', the following statement will work:
SELECT * FROM `table` GROUP BY id HAVING COUNT(id) = 1
This will filter out duplicate results entirely (e.g. it'll take out your 2's and 6's)
Three ways. One with GROUP BY and HAVING:
SELECT columnX
FROM tableX
GROUP BY columnX
HAVING COUNT(*) = 1 ;
one with a correlated NOT EXISTS subquery:
SELECT columnX
FROM tableX AS t
WHERE NOT EXISTS
( SELECT *
FROM tableX AS t2
WHERE t2.columnX = t.columnX
AND t2.pk <> t.pk -- pk is the primary key of the table
) ;
and an improvement on the first way (if you have a primary key pk column and an index on (columnX, pk):
SELECT columnX
FROM tableX
GROUP BY columnX
HAVING MIN(pk) = MAX(pk) ;
select id from foo group by id having count(*) < 2;
I had a problem in database. I have to insert duplicate records of a particular record on a another table based on a value.
First i used cursor to fetch each records and get the number of duplication i wants and after that used another cursor for duplication. Everything worked fine. But if the records in more than 500, i went dead slow. Then i did some research and found a way to insert without cursor.
INSERT INTO report(id, Name)
SELECT i.id,i.Name FROM (SELECT 1 AS id
UNION SELECT 2
UNION SELECT 3
UNION SELECT 4
UNION SELECT 5
UNION SELECT 6
UNION SELECT 7
UNION SELECT 8
UNION SELECT 9
UNION SELECT 10) AS o
INNER JOIN table i WHERE o.id<=i.frequence;
where frequence is the number of duplication. Please drop your idea to improve your query.
You could try creating a table with a record for each value from 1 to 10 and then join to that. I'm not sure it would be any faster though. You would have to experiment with it.
In this example the table with the values from 1 to 10 is called "dup" and the field containing these values is called "id".
INSERT INTO report(id, Name)
SELECT i.id, i.Name
FROM table i
JOIN dup d
ON d.id <= i.frequence;
If you have any table that contains a row number that goes at least as high as the maximum frequence, you could to this:
INSERT INTO report(id, Name)
SELECT i.id,i.Name FROM table i
inner join (
select distinct some_row_number_column from some_table
) o on o.some_row_number_column <= i.frequence;
This is basically the same as what you were doing, but it avoids the messy union all statements.
Or you could make a cursor that inserts numbers from 1 to the maximum frequence into a temporary table, then use that in your join. Or you could use a row numbering variable to generate the necessary sequence. Basically, do anything that will generate a list of consecutive numbers from 1 to the maximum that you need.
I would normally use recursion for this (DB2 syntax):
INSERT INTO report(id, Name)
with num_list (num) as (
values (1)
union all
select num + 1 from num_list
where num < (select max(frequence) from table)
)
SELECT i.id,i.Name FROM table i
inner join num_list on num_list.num <= i.frequence;
However, MySQL doesn't support recursion, apparently.
I just need to know if a query returns or not a record.
Of course I can do this:
SELECT COUNT(*) FROM tbl WHERE conds;
But this returns the exact number of rows (of course), and I don't need this overhead.
So I thought this query:
SELECT COUNT(*) FROM (SELECT id FROM tbl WHERE conds LIMIT 1) as t1
Limiting the internal query to 1.
Is this faster? Or considering I am doing a subquery it cancels the benefits of LIMIT 1?
Note: for everyone asking theirself, I can't apply LIMIT 1 to the first query because it doens't work
The inner-select in the second query is redundant.
If you just want to check at-least of one row :-
SELECT 1 FROM tbl // return 1
WHERE conds // depends on your index and query
ORDER BY NULL // avoid file-sort
LIMIT 1; // minimum row
Why not just:
SELECT 1 FROM tbl WHERE conds LIMIT 1
You could do:
SELECT 1 WHERE EXISTS(SELECT id FROM tbl WHERE CONDITION)
Or something like:
SELECT 1 WHERE EXISTS (SELECT id FROM tbl WHERE id IN( 1000, 1001))