i have the following sql query
SELECT id,
title,
total_likes,
IFNULL(SELECT 1 FROM 'likedata' WHERE user_id=$UID AND post_id=posts.id)0) AS is_liked
FROM 'posts'
I want to create mysql function to make my query bit shorter,
i don't know how to wrap the second query into sql function and pass 2 variables ($UID and $PID) to function to make this query shorter and more understandable.
any help would be great, thanks in advance
This isn't a good use case for a function.
A more fluent way of expressing this in SQL would be to use a left outer join, e.g:
SELECT posts.id, posts.title, posts.total_likes,
(likedata.id IS NOT NULL) AS is_liked
FROM posts
LEFT JOIN likedata ON (
posts.id = likedata.post_id AND likedata.user_id = $UID
)
You can see this in action here.
You can actually simply use exists. MySQL treats booleans as numbers, so:
SELECT p.id, p.title, p.total_likes,
( EXISTS (SELECT 1 FROM likedata ld WHERE ld.post_id = p.id AND ld.user_id = $UID)
) as is_liked
FROM posts p;
This seems closest to your original intent.
#duskwuff's answer is also a typical way to approach this. There is a slight different, because duplicates in likedata would result in duplicate rows in the result set using a join.
For either form, you want an index on likedata(post_id, user_id). And if you are passing $UID in, you should be using parameterized queries.
Related
I want to optimize these SQL queries using if-else but how I should use it? .
if this query result contain 'ALL'
SELECT
bdsubcategory.subcategoryID as ID,
bdsubcategory.subcategoryName as Name
FROM
phonebook.newsms_subscription
INNER JOIN bdsubcategory ON bdsubcategory.subcategoryID = newsms_subscription.subcategoryID
INNER JOIN newsms_client ON newsms_subscription.clientID =newsms_client.clientID
INNER JOIN newsms_person ON newsms_subscription.personID = newsms_person.personID
WHERE
newsms_subscription.isActive = 1 AND
newsms_person.personID = '856'
Then i want to query this
SELECT
bdsubcategory.subcategoryID as ID,
bdsubcategory.subcategoryName as Name
FROM
phonebook.newsms_subscription
INNER JOIN bdsubcategory ON bdsubcategory.subcategoryID = newsms_subscription.subcategoryID
INNER JOIN newsms_person ON newsms_subscription.personID = newsms_person.personID
WHERE
newsms_subscription.isActive = 1
GROUP BY subcategoryName
ORDER BY subcategoryName
otherwise take query1 result .
The problem is that if we do not refactor your project, then you always have to evaluate query1 and see whether it contains All or not. If it does not contain All, then you need to evaluate query2 as well. This can hardly be optimized, let's see a few approaches:
Quickening query1
Since All might be not be the very last evaluated element, adding it to the filter and limiting it is a good idea to quicken query1:
SELECT
COUNT(*)
FROM
phonebook.newsms_subscription
INNER JOIN bdsubcategory ON bdsubcategory.subcategoryID = newsms_subscription.subcategoryID
INNER JOIN newsms_client ON newsms_subscription.clientID =newsms_client.clientID
INNER JOIN newsms_person ON newsms_subscription.personID = newsms_person.personID
WHERE
newsms_subscription.isActive = 1 AND
newsms_person.personID = '856' AND
bdsubcategory.subcategoryName = 'ALL'
LIMIT 0, 1
So, you could create a stored procedure which evaluates query1' (query1' is the quickened version of query1, as seen above) and if there is a result, then we need to execute query1. Otherwise we need to execute query2. This way you still execute two queries, but the first query is optimized.
Refactoring
Note that the second query does not change. You could create a table where you could cache its results, using a periodic job. Then, you could skip the second table to
SELECT ID, Name
FROM MyNewTable;
without the many joins. You would also cache the results of the first query into a table where the items having ALL would be stored and query that table.
One option would be to use a CASE.
Change this:
newsms_person.personID = '856'
To this:
'Y' = CASE WHEN UPPER('856') = 'ALL' THEN 'Y'
WHEN newsms_person.personID = '856' THEN 'Y'
ELSE 'N' END
Alternatively, a stored procedure could be used to first validate whether the personID seems valid, then returns the appropriate data.
I have a table with exchange rate like below
And I am using the maxofdate to pick all these values based on currency code. But the query is giving blank.
Select USDAMOUNT * dbo.EXCHANGERATEAMT
from dbo.Amount_monthly
Left Join dbo.EXCHANGERATE on dbo.Amount_monthly.Currencycode=dbo.EXCHANGERATE.fromcurrencycode
WHERE ValidToDateTime = (Select MAX(ValidToDateTime) from dbo.EXCHANGERATE)
AND dbo.EXCHANGERATE.EXCHANGERATETYPECODE = 'DAY'
Using this statement
CONVERT(DATE,ValidToDateTime) = CONVERT(DATE,GETDATE()-1)
instead of subquery is giving me expected result.
Can someone correct this.
thanks in advance.
If I understand correctly, you need two things. First, the condition for the max() needs to match the condition in the outer query. Second, if you really want a left join, then conditions on the second table need to go in the on clause.
The resulting query looks like:
Select . . .
from dbo.Amount_monthly am Left Join
dbo.EXCHANGERATE er
on am.Currencycode = er.fromcurrencycode and
er.ValidToDateTime = (Select max(er2.ValidToDateTime)
from dbo.EXCHANGERATE er2
where er2.EXCHANGERATETYPECODE = 'DAY'
) and
er.EXCHANGERATETYPECODE = 'DAY';
I would write this using window functions, but that is a separate issue.
Try removing WHERE clause for ValidToDateTime and include it in the JOIN as AND condition
SELECT USDAMOUNT * dbo.EXCHANGERATEAMT
FROM dbo.Amount_monthly
LEFT JOIN dbo.EXCHANGERATE
ON dbo.Amount_monthly.Currencycode = dbo.EXCHANGERATE.fromcurrencycode
AND ValidToDateTime = (SELECT MAX(ValidToDateTime) --remove WHERE clause
FROM dbo.EXCHANGERATE)
AND dbo.EXCHANGERATE.EXCHANGERATETYPECODE = 'DAY';
I cleaned up your query a bit: as the other folks mentioned you needed to close the parentheses around the MAX(Date) sub-query, and if you reference a LEFT JOINed table in the WHERE clause, it behaves like an INNER JOIN, so I changed to in INNER. You also had "dbo" sprinkled in as a field prefix, but that (the namespace) only prefixes a database, not a field. I added the IS NOT NULL check just to avoid SQL giving the "null values were eliminated" SQL warning. I used the aliases "am" for the first table and "er" for the 2nd, which makes it more readable:
SELECT am.USDAMOUNT * er.EXCHANGERATEAMT
FROM dbo.Amount_monthly am
JOIN dbo.EXCHANGERATE er
ON am.Currencycode = er.fromcurrencycode
WHERE er.ValidToDateTime = (SELECT MAX(ValidToDateTime) FROM dbo.EXCHANGERATE WHERE ValidToDateTime IS NOT NULL)
AND er.EXCHANGERATETYPECODE = 'DAY'
If you're paranoid like I am, you might also want to make sure the exchange rate is not zero to avoid a divide-by-zero error.
I know this has been asked plenty times before, but I cant find an answer that is close to mine.
I have the following query:
SELECT c.cases_ID, c.cases_status, c.cases_title, ci.custinfo_FName, ci.custinfo_LName, c.cases_timestamp, o.organisation_name
FROM db_cases c, db_custinfo ci, db_organisation o
WHERE c.userInfo_ID = ci.userinfo_ID AND c.cases_status = '2'
AND organisation_name = (
SELECT organisation_name
FROM db_sites s, db_cases c
WHERE organisation_ID = '111'
)
AND s.sites_site_ID = c.sites_site_ID)
What I am trying to do is is get the cases, where the sites_site_ID which is defined in the cases, also appears in the db_sites sites table alongside its organisation_ID which I want to filter by as defined by "organisation_ID = '111'" but I am getting the response from MySQL as stated in the question.
I hope this makes sense, and I would appreciate any help on this one.
Thanks.
As the error states your subquery returns more then one row which it cannot do in this situation. If this is not expect results you really should investigate why this occurs. But if you know this will happen and want only the first result use LIMIT 1 to limit the results to one row.
SELECT organisation_name
FROM db_sites s, db_cases c
WHERE organisation_ID = '111'
LIMIT 1
Well the problem is, obviously, that your subquery returns more than one row which is invalid when using it as a scalar subquery such as with the = operator in the WHERE clause.
Instead you could do an inner join on the subquery which would filter your results to only rows that matched the ON clause. This will get you all rows that match, even if there is more than one returned in the subquery.
UPDATE:
You're likely getting more than one row from your subquery because you're doing a cross join on the db_sites and db_cases table. You're using the old-style join syntax and then not qualifying any predicate to join the tables on in the WHERE clause. Using this old style of joining tables is not recommended for this very reason. It would be better if you explicitly stated what kind of join it was and how the tables should be joined.
Good pages on joins:
http://dev.mysql.com/doc/refman/5.0/en/join.html (for the right syntax)
http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html (for the differences between the types of joins)
I was battling this for an hour, and overcomplicated it completely. Sometimes a quick break and writing it out on an online forum can solve it for you ;)
Here is the query as it should be.
SELECT c.cases_ID, c.cases_status, c.cases_title, ci.custinfo_FName, ci.custinfo_LName, c.cases_timestamp, c.sites_site_ID
FROM db_cases c, db_custinfo ci, db_sites s
WHERE c.userInfo_ID = ci.userinfo_ID AND c.cases_status = '2' AND (s.organisation_ID = '111' AND s.sites_site_ID = c.sites_site_ID)
Let me re-write what you have post:
SELECT
c.cases_ID, c.cases_status, c.cases_title, ci.custinfo_FName, ci.custinfo_LName,
c.cases_timestamp, c.sites_site_ID
FROM
db_cases c
JOIN
db_custinfo ci ON c.userInfo_ID = ci.userinfo_ID and c.cases_status = '2'
JOIN
db_sites s ON s.sites_site_ID = c.sites_site_ID and s.organization_ID = 111
Consider following two tables:
tag_names (tag_id, tag_name)
tag_links (tag_id, image_id)
An image can have multiple tags, I want to select all tags for a specific image id.
I am trying following query, but it doesnt seem to select correctly (selects only one row), What is wrong with it?
SELECT tag_name
FROM tag_names
LEFT JOIN tag_links.tag_id = tag_names.tag_id
WHERE tag_links.image_id = $image_id
Edit: I'm using CodeIgniter Active record query, but I wrote in basic SQL format so that if someone is not fimiliar with CodeIgniter can help. However, this query works fine with simple mysql format (without using CodeIgniter) but strangely does not work with CodeIgniter, even there is no any problem with the syntax, it just selects one row.
Here is CodeIgniter Syntax:
$this->db->select('tag_name');
$this->db->from('tag_names');
$this->db->join('tag_links', 'tag_links.tag_id = tag_names.tag_id', 'left');
$this -> db -> where('tag_links.image_id', (int)$image_id);
$query = $this->db->get();
Try this:
SELECT tag_name
FROM tag_names
LEFT JOIN tag_links
ON tag_links.tag_id = tag_names.tag_id
WHERE tag_links.image_id = $image_id
IMHO you forgot to join table (properly with ON statement) you are using.
EDIT: I have 2 ideas how to get rid of the problem:
First:
Change the line with SELECT
$this->db->select('tag_names.tag_name');
Second:
Use select() function with complete query:
$this->db->select($query, false);
$this->db->select() accepts an optional second parameter. If you set
it to FALSE, CodeIgniter will not try to protect your field or table
names with backticks. This is useful if you need a compound select
statement.
from: http://codeigniter.com/user_guide/database/active_record.html#select
It seems that you have a syntax error (you forgot tag_links in JOIN clause). By the way in my opinion you don't need LEFT JOIN for this purpose otherwise you may get incorrect results.
SELECT tag_name
FROM
tag_names
JOIN tag_links ON tag_links.tag_id = tag_names.tag_id
WHERE tag_links.image_id = $image_id
SELECT tag_names.tag_name
FROM tag_links
LEFT JOIN tag_names.tag_id = tag_links.tag_id
WHERE tag_links.image_id = $image_id
tag_names is only going to have single entry for a given ID, which means your query will return a single result. You need to primarily select from tag_links and then join the name of the tag on top of it, so you correctly select from the table with the multiple entries.
I'm trying to use the following query, and if it only has one having statement it works as expected. If I add the second having statement it does not work.
SELECT candidate.first_name,
candidate.last_name,
qualification.code,
property.value AS funding_band_value,
qualification.funding_band,
property.value AS qualification_level_value,
qualification.qualification_level_id
FROM candidate_qualification, candidate, qualification, property
WHERE candidate_qualification.candidate_id=candidate.id and
candidate_qualification.qualification_id=qualification.id
HAVING funding_band_value = (select property.value from property where qualification.funding_band=property.id) and
HAVING qualification_level_value = (select property.value from property where qualification.qualification_level_id=property.id)
Could someone explain why this doesn't work and how I should do this.
HAVING acts similarly to WHERE or GROUP BY. You reference it once to start using it and combine multiple statements with AND or OR operators. An in depth look at the query parser might give you a more explicit answer.
You don't need HAVING here, just use AND so it is part of your WHERE clause.
The subqueries are not necessary, those tables are already joined.
Something like this should be closer to what you want:
SELECT c.first_name,
c.last_name,
q.code,
p.value AS funding_band_value,
q.funding_band,
p.value AS qualification_level_value,
q.qualification_level_id
FROM candidate_qualification cq
INNER JOIN candidate c ON cq.candidate_id=c.id
INNER JOIN qualification q ON cq.qualification_id=q.id
INNER JOIN property p ON q.funding_band=p.id
and q.qualification_level_id=p.id