I have database with schema on picture below and I need to select everything related to one row (one id) of [letaky]. That means the related [zamestnanci], every related [obsah] and every [knihy] in it.
This is the first time i used relations in database and i have no idea how to make such a select.
Use JOIN ... ON:
SELECT *
FROM zamestnanci
JOIN lekaty ON lekaty.zamestnanciid = zamestnanci.id
JOIN obsah ON obsah.idletaku = lekaty.id
JOIN knihy ON knihy.id = obsah.idknihy
WHERE letaky.id = 123
You may also want to consider whether you need INNER JOIN, LEFT JOIN or RIGHT JOIN for each of these joins. The difference between these JOINs is described in many other questions on StackOverflow, for example this one:
SQL Join Differences
Related
I have two tables, I've been trying to print the result from each but they are being duplicated. These are the two MySQL tables and the result. Notice the duplication.
The sql code for the project is:
SELECT * FROM savings,savtype WHERE cust_id=".$_SESSION['user']
I'm also looking for a work around this, in the meantime, id appreciate any assistance on this.
because you are not specifying how the two tables are related. You need to add that, either via an explicit ... JOIN ... (USING|ON)
SELECT
*
FROM
savings JOIN savtype USING (savtype_id)
WHERE
cust_id = ".$_SESSION['user']
or by providing the criteria in the where clause.
SELECT
*
FROM
savings, savtype
WHERE
savings.savtype_id = savtype.savtype_id AND
cust_id = ".$_SESSION['user']
As I understand from the screenshot you added, it makes joint between those tables, and what you probably want it left join from savings and savtype tables.
SELECT *
FROM `savings`
LEFT JOIN `savtype`
ON savings.savtype_id=savtype.savtype_id
where cust_id=".$_SESSION['user'] .";
Update if this did the trick,
You can learn more about left join here: https://www.w3schools.com/sql/sql_join_left.asp
I have two tables usersin and usersout(I can not change schema, a lot of system changes must be done in php otherwise). I should get all user records in a query but I should mark them if they are in or out also a user may have an in record and out record I shouldn't show in record if has an out record.
I have created tables with sample data in SQL Fiddle: http://www.sqlfiddle.com/#!9/ac99a/1/0
Can u help me how can I remove duplicates of user records in this union query?
If you want to have all entries with an entry in either the in or out table, but not in both of them, then a full outer join would be your friend.
Since MySQL does not know that kind of join, you can emulate it with a left outer join and a right outer join combined like so:
SELECT
ui.id, ui.user, 'i'
FROM
usersIN ui
LEFT OUTER JOIN
usersOUT uo ON ui.user = uo.user
WHERE uo.id IS NULL
UNION
SELECT
uo.id, uo.user, 'o'
FROM
usersIN ui
RIGHT OUTER JOIN
usersOUT uo ON ui.user = uo.user
WHERE ui.id IS NULL;
This should give you the right output.
A good visual explanation of joins can be found here
I'm working through the JOIN tutorial on SQL zoo.
Let's say I'm about to execute the code below:
SELECT a.stadium, COUNT(g.matchid)
FROM game a
JOIN goal g
ON g.matchid = a.id
GROUP BY a.stadium
As it happens, it produces the same output as the code below:
SELECT a.stadium, COUNT(g.matchid)
FROM goal g
JOIN game a
ON g.matchid = a.id
GROUP BY a.stadium
So then, when does it matter which table you assign at FROM and which one you assign at JOIN?
When you are using an INNER JOIN like you are here, the order doesn't matter. That is because you are connecting two tables on a common index, so the order in which you use them is up to you. You should pick an order that is most logical to you, and easiest to read. A habit of mine is to put the table I'm selecting from first. In your case, you're selecting information about a stadium, which comes from the game table, so my preference would be to put that first.
In other joins, however, such as LEFT OUTER JOIN and RIGHT OUTER JOIN the order will matter. That is because these joins will select all rows from one table. Consider for example I have a table for Students and a table for Projects. They can exist independently, some students may have an associated project, but not all will.
If I want to get all students and project information while still seeing students without projects, I need a LEFT JOIN:
SELECT s.name, p.project
FROM student s
LEFT JOIN project p ON p.student_id = s.id;
Note here, that the LEFT JOIN refers to the table in the FROM clause, so that means ALL of students were being selected. This also means that p.project will be null for some rows. Order matters here.
If I took the same concept with a RIGHT JOIN, it will select all rows from the table in the join clause. So if I changed the query to this:
SELECT s.name, p.project
FROM student s
RIGHT JOIN project p ON p.student_id = s.id;
This will return all rows from the project table, regardless of whether or not it has a match for students. This means that in some rows, s.name will be null. Similar to the first example, because I've made project the outer joined table, p.project will never be null (assuming it isn't in the original table). In the first example, s.name should never be null.
In the case of outer joins, order will matter. Thankfully, you can think intuitively with LEFT and RIGHT joins. A left join will return all rows in the table to the left of that statement, while a right join returns all rows from the right of that statement. Take this as a rule of thumb, but be careful. You might want to develop a pattern to be consistent with yourself, as I mentioned earlier, so these queries are easier for you to understand later on.
When you only JOIN 2 tables, usually the order does not matter: MySQL scans the tables in the optimal order.
When you scan more than 2 tables, the order could matter:
SELECT ...
FROM a
JOIN b ON ...
JOIN c ON ...
Also, MySQL tries to scan the tables in the fastest way (large tables first). But if a join is slow, it is possible that MySQL is scanning them in a non-optimal order. You can verify this with EXPLAIN. In this case, you can force the join order by adding the STRAIGHT_JOIN keyword.
The order doesn't always matter, I usually just order it in a way that makes sense to someone reading your query.
Sometime order does matter. Try it with LEFT JOIN and RIGHT JOIN.
In this instance you are using an INNER JOIN, if you're expecting a match on a common ID or foreign key, it probably doesn't matter too much.
You would however need to specify the tables the correct way round if you were performing an OUTER JOIN, as not all records in this type of join are guaranteed to match via the same field.
yes, it will matter when you will user another join LEFT JOIN, RIGHT JOIN
currently You are using NATURAL JOIN that is return all tables related data, if JOIN table row not match then it will exclude row from result
If you use LEFT / RIGHT {OUTER} join then result will be different, follow this link for more detail
I have a real mindbender of a MySQL problem which I am now thinking there is no answer to. Please help me, you are my only hope!
Stripping it down to the basics, I have two tables, "People" and "Activity". It is possible (long story and lots of data involved) for these two tables to be joined by two different relationship tables: people_activity and entity_activity
I need to do a query on the activity table which gets the people record/s linked to activity records based on both relationship tables.
This is what I have, but it is massively slow on lots of data:
select * from activity
left join peopleactivity on peopleactivity.activityid = activity.activityid
left join entityactivity on entityactivity.activityid = activity.activityid
left join people on (peopleactivity.peopleid = people.peopleid OR
entityactivity.entityid = people.peopleid)
Some more notes - I have also tried creating a view to combine the results of the two relationship tables and instead joining people and activity via this view. This also works, but is also still massively slow
Changing how the relationship/s work to consolodate to one table is a major headache
I have also tried a union -like this -
select * from activity
left join peopleactivity on peopleactivity.activityid = activity.activityid
left join people on (peopleactivity.peopleid = people.peopleid)
union
select * from activity
left join peopleactivity on peopleactivity.activityid = activity.activityid
left join people on (entityactivity.entityid= people.peopleid)
which also works, but for other reasons causes me problems. I really need to do this in one query without changing too much underlying.
Has anyone got any super amazing ideas that I have missed??!
You may try to replace OR with IN
left join people on people.peopleid IN (peopleactivity.peopleid, entityactivity.entityid)
1.) Try setting the id of the tables as the primary key on each table
2.) Use inner joins instead of left joins. Not sure why you are using left joins here as you will get all the results of the other tables left joined on the activity table and get basically all records whether or not they have a join value in another table. I think this might also help you. Can you post a describe of your tables.
I think you should keep the UNION query but making those INNER joins. Do you really need LEFT joins?
You could also change it into UNION ALL, which will have some performance gain:
SELECT activity.*, people.*, 'PA' AS joining_table
FROM activity
JOIN peopleactivity ON peopleactivity.activityid = activity.activityid
JOIN people ON peopleactivity.peopleid = people.peopleid
UNION ALL
SELECT activity.*, people.*, 'EA'
FROM activity
JOIN entityactivity ON entityactivity.activityid = activity.activityid
JOIN people ON entityactivity.entityid = people.peopleid
Thanks for the comments. I had tried various incarnations of the above. My answer was to set up a new table, copy all the existing links into that table, and then use triggers to add/remove links to that table whenever the links were added removed in the two separate link tables. This works well and also allows me to use indexes on this new table to keep things nice and snappy. Many thanks for those that took the time to post the ideas though!
I know the usage of joins, but sometimes I come across such a situation when I am not able to decide which join will be suitable, a left or right.
Here is the query where I am stuck.
SELECT count(ImageId) as [IndividualRemaining],
userMaster.empName AS ID#,
CONVERT(DATETIME, folderDetails.folderName, 101) AS FolderDate,
batchDetails.batchName AS Batch#,
Client=#ClientName,
TotalInloaded = IsNull(#TotalInloaded,0),
PendingUnassigned = #PendingUnassigned,
InloadedAssigned = IsNull(#TotalAssigned,0),
TotalProcessed = #TotalProcessed,
Remaining = #Remaining
FROM
batchDetails
Left JOIN folderDetails ON batchDetails.folderId = folderDetails.folderId
Left JOIN imageDetails ON batchDetails.batchId = imageDetails.batchId
Left JOIN userMaster ON imageDetails.assignedToUser = userMaster.userId
WHERE folderDetails.ClientId =#ClientID and verifyflag='n'
and folderDetails.FolderName IN (SELECT convert(VARCHAR,Value) FROM dbo.Split(#Output,','))
and userMaster.empName <> 'unused'
GROUP BY userMaster.empName, folderDetails.folderName, batchDetails.batchName
Order BY folderDetails.Foldername asc
Yes, it depends on the situation you are in.
Why use SQL JOIN?
Answer: Use the SQL JOIN whenever multiple tables must be accessed through an SQL SELECT statement and no results should be returned if there is not a match between the JOINed tables.
Reading this original article on The Code Project will help you a lot: Visual Representation of SQL Joins.
Also check this post: SQL SERVER – Better Performance – LEFT JOIN or NOT IN?.
Find original one at: Difference between JOIN and OUTER JOIN in MySQL.
In two sets:
Use a full outer join when you want all the results from both sets.
Use an inner join when you want only the results that appear in both
sets.
Use a left outer join when you want all the results from set a, but
if set b has data relevant to some of set a's records, then you also
want to use that data in the same query too.
Please refer to the following image:
I think what you're looking for is to do a LEFT JOIN starting from the main-table to return all records from the main table regardless if they have valid data in the joined ones (as indicated by the top left 2 circles in the graphic)
JOIN's happen in succession, so if you have 4 tables to join, and you always want all the records from your main table, you need to continue LEFT JOIN throughout, for example:
SELECT * FROM main_table
LEFT JOIN sub_table ON main_table.ID = sub_table.main_table_ID
LEFT JOIN sub_sub_table on main_table.ID = sub_sub_table.main_table_ID
If you INNER JOIN the sub_sub_table, it will immediately shrink your result set down even if you did a LEFT JOIN on the sub_table.
Remember, when doing LEFT JOIN, you need to account for NULL values being returned. Because if no record can be joined with the main_table, a LEFT JOIN forces that field to appear regardless and will contain a NULL. INNER JOIN will obviously just "throw away" the row instead because there's no valid link between the two (no corresponding record based on the ID's you've joined)
However, you mention you have a where statement that filters out the rows you're looking for, so your question on the JOIN's are null & void because that is not your real problem. (This is if I understand your comments correctly)