I have that tables schema
via linq2Sql, I generate a query:
var types = (from d in TabD
where d.SomeID == myID
group d by d.TabC.TabB.TabA into g
select g.Key);
which generates that query
SELECT Distinct1.ID, Distinct1.Name
FROM (
SELECT DISTINCT ss.ID, ss.Name
FROM TabD AS d
INNER JOIN TabC AS c ON d.TabC_ID = c.ID
INNER JOIN TabB AS b ON c.TabB_ID = b.ID
LEFT OUTER JOIN TabA AS a ON b.TabA_ID = a.ID
WHERE sg.SomeID = 1
) AS Distinct1
which works, but why there is LEFT OUTER JOIN instead of INNER JOIN ?
You get LEFT JOIN whenever the foreign key on a table is nullable. If your database defines TabA_ID as TabA_ID int NULL it means TabB reference to TabA is not enforced.
Since it is not enforced based on your schema LINQ2SQL generates a LEFT join.
Hope this helps
Related
I'm currently reading a query that I don't usually see in some other queries when joining tables.
SELECT
*
FROM table_1
RIGHT OUTER JOIN table_2
INNER JOIN table_3
ON table_2.some_id = table_3.some_id
RIGHT OUTER JOIN table_4
ON table_3.some_id = table_4.some_id
LEFT OUTER JOIN table_5
ON table_4.some_id = table_5.some_id
ON table_1.some_id = table_4.some_id
Please don't mind how those tables being joined. I just want to know how did that query works? Thank you in advance.
I think the SQL Server documentation captures what is happening better than MySQL. (Here is the documentation.)
This is a parsing issue, as Laurence has observed. You can understand what is by looking at the syntax diagram for the from statement. There is a recursive reference that most people never think about (including me). A joined table has the following syntax:
<joined_table> ::=
{
<table_source> <join_type> <table_source> ON <search_condition>
| <table_source> CROSS JOIN <table_source>
| left_table_source { CROSS | OUTER } APPLY right_table_source
| [ ( ] <joined_table> [ ) ]
}
The key here is the first piece, <table_source> <join_type> <table_source>. Well, guess what, a table_source can be a joined_table (as well as a bunch of other things). This means that the syntax:
A join B join C ON <condition1> ON <condition2>
Fits the grammar above and is interpreted as:
A join (B join C on <condition1>) ON <condition2>
That is, the expression B join C on <condition1> is treated as a "table_source".
My guess is that if both SQL Server and MySQL do it this way, then it is probably part of the standard. However, the standard is a bit harder to understand than SQL Server's syntax diagrams.
I think this is down to the way right outer join is evaluated:
Select
*
from
a
right outer join
b
inner join
c
on b.id = c.id
on a.id = b.id;
is evaluated as
Select
*
from
a
right outer join (
b
inner join
c
on b.id = c.id
)
on a.id = b.id;
Mixing left and right outer join is a path to madness.
The OP's query seems to be driven by a desire to exclude parenthesis, or derived tables. It is equivalent to:
Select
a.id ida,
b.id idb,
c.id idc,
d.id idd,
e.id ide
From
D
left outer join
A
on d.id = a.id
left outer join
E
on d.id = e.id
left outer join (
B
inner join
C
on b.id = c.id
)
on d.id = b.id;
Example SQLFiddle
I'm having a real mind blank - This code selects rows quite nicely apart from those entries where I change st.station_id from a value of '1' to a different (but still valid) number but where there are no entries for that station_id in either the station_owner_map table, the organisation table or the cap_gen_data_table. I basically need to amend my sql to ignore any table where there are no entries.
Select st.station_id, st.station_name , st.st_town, st.st_state, c1.country_name, o1.organisation_name, som1.equity, st.river_basin, st.cost, st.cost_ref, st.comm_year,cg1.caporgen, ht1.hydro_name, cg1.value, srs1.srs_description, cg1.ref_year
FROM station st
inner join station_country_map scm1 on st.station_id = scm1.station_id
inner join country c1 on scm1.country_id = c1.country_id
inner join station_owner_map som1 on st.station_id = som1.station_id
inner join organisation o1 on som1.owner_id = o1.org_id
inner join cap_gen_data cg1 on st.station_id = cg1.station_id
inner join value_lookup vl1 on cg1.caporgen = vl1.id
inner join hydro_type ht1 on cg1.hydro_type_id = ht1.type_id
inner join station_record_status srs1 on cg1.capacity_status = srs1.st_rec_stat_id
where st.station_id = 1
It's caused by your inner joins. Inner join means there has to be a value in both tables for the record to show up in the result set.
Use left join instead, then only the table 'on the left' has to have a value.
Use left join on tables where the value may not be present.
If you have two tables A and B an inner join will only return the rows from A where the join condition is met. A left join will return all rows from A regardless of if the join condition is satisfied. Columns in the select statement associated with B will be null when a left join is used.
I have only added the left join to the tables you have indicated. If other tables may not satisfy the join condition change the join type from inner to left.
Select st.station_id, st.station_name , st.st_town, st.st_state, c1.country_name, o1.organisation_name, som1.equity, st.river_basin, st.cost, st.cost_ref, st.comm_year,cg1.caporgen, ht1.hydro_name, cg1.value, srs1.srs_description, cg1.ref_year
FROM station st
inner join station_country_map scm1 on st.station_id = scm1.station_id
inner join country c1 on scm1.country_id = c1.country_id
left join station_owner_map som1 on st.station_id = som1.station_id
left join organisation o1 on som1.owner_id = o1.org_id
left join cap_gen_data cg1 on st.station_id = cg1.station_id
inner join value_lookup vl1 on cg1.caporgen = vl1.id
inner join hydro_type ht1 on cg1.hydro_type_id = ht1.type_id
inner join station_record_status srs1 on cg1.capacity_status = srs1.st_rec_stat_id
where st.station_id = 1
Is it possible to INNER JOIN a MySQL query to achieve this result?
I have a table with Strategies and a table with Members. The Strategy table holds the ID of the author that corresponds to their ID in the Member table and the ID of an author that updated the existing author's work. Is it possible to grab a reference to both of these people at the same time? Something like the following, which returns no errors, but also no results...
SELECT * FROM Strategies
INNER JOIN Members AS a
INNER JOIN Members AS b
WHERE Strategies.ID='2'
AND Strategies.AuthorID = a.ID
AND Strategies.UpdateAuthorID = b.ID
Use a LEFT JOIN:
SELECT
s.*,
a.Name AS MemberName,
b.Name AS UpdatedMemberName
FROM Strategies AS s
LEFT JOIN Members AS a ON s.AuthorID = a.ID AND s.ID = 2
LEFT JOIN Members AS b ON s.UpdateAuthorID = b.ID AND s.ID = 2 ;
If you want them in one column use COALESCE:
SELECT
s.*,
COALESCE(a.Name, b.Name) AS MemberName
FROM Strategies AS s
LEFT JOIN Members AS a ON s.AuthorID = a.ID AND s.ID = 2
LEFT JOIN Members AS b ON s.UpdateAuthorID = b.ID AND s.ID = 2
SELECT toD.dom_url AS ToURL,
fromD.dom_url AS FromUrl,
rvw.*
FROM reviews AS rvw
LEFT JOIN domain AS toD
ON toD.Dom_ID = rvw.rev_dom_for
LEFT JOIN domain AS fromD
ON fromD.Dom_ID = rvw.rev_dom_from
if domain is table name
I have a MySQL JOIN query where 2 tables are joined to get the output
select distinct (a.error_type),a.links_id, a.crawl_cycle , b.* from $table a inner join crawler_error_type b on a.error_type = b.error_type where a.projects_id = '$pid' and b.error_priority_page_level='High'
Now I want to check if the value of the field error_type is present in the third table. If it is present then it shouldn't give me the resulted row.
Add the below:
LEFT OUTER JOIN third_table c ON c.error_type = a.error_type
and
WHERE c.error_type is null
LEFT OUTER JOIN will display all the records from a table joining on third_table. Since you do not want the record with matching error type from third_table, use WHERE c.error_type is null
You need to add one more INNER JOIN on third_table as:
SELECT DISTINCT a.error_type, a.links_id, a.crawl_cycle , b.*
FROM $table a
INNER JOIN crawler_error_type b
ON a.error_type = b.error_type
INNER JOIN third_table c
ON a.error_type = c.error_type
WHERE a.projects_id = '$pid' AND
b.error_priority_page_level='High'.
If I understand it correctly, you should just be able to inner join the third table. Inner joins will on show the record if they have records in the joining tables. If I am misunderstanding could you please elaborate. Thx
select distinct (a.error_type),a.links_id, a.crawl_cycle , b.*
from $table a
inner join crawler_error_type b on a.error_type = b.error_type
inner join some_table_3 c on c.error_type = a.error_type
where a.projects_id = '$pid' and b.error_priority_page_level='High'
I'm coming across this situation alot, I'll have a query that will have one table needed in a join condition that may have no entries therefore requiring me to use a LEFT JOIN. I can't wrap my head around the syntax when it's used with more than 1 join.
I'll have:
SELECT A.*, B.*, C.*
FROM A, B, C
WHERE A.id = C.id
AND C.aid = A.id
AND B.cid = C.id
Along comes D with the possibility of being empty and I have to rewrite the query and run into problems.
How can I simply join D to any one of these tables?
You're much better off explicitly specifying all of your JOINs. That should make things much clearer.
SELECT A.*, B.*, C.*, D.*
FROM A
INNER JOIN C
ON C.aid = A.id
INNER JOIN B
ON B.cid = C.id
LEFT JOIN D
ON C.did = d.id
My advice is to never specify more than one column on FROM clause.
For clarity, it's better to always:
Use JOIN clause
Use aliases
Specify columns of joined tables on left side of equal sign
Example:
SELECT a.*, b.*, c.*
FROM ATable a
INNER JOIN BTable b
ON b.id = a.id
INNER JOIN CTable c
ON c.id = a.id
WHERE a.someColumn = 'something'
Not sure about MySQL, but in some other SQL flavors, you can use the same on UPDATES and DELETES, like:
DELETE FROM a
FROM ATable a
INNER JOIN BTable b
ON b.id = a.id
INNER JOIN CTable c
ON c.id = a.id
WHERE a.someColumn = 'something'
or
UPDATE a
SET something = newValue
FROM ATable a
INNER JOIN BTable b
ON b.id = a.id
INNER JOIN CTable c
ON c.id = a.id
WHERE a.someColumn = 'something'
The syntax below should help you. The basic premise is whatever table is listed LEFT is the required.. the table (or alias) on the right is optional. I understand you don't quite get it, and your syntax sample shows that (not meant to criticize) as you are joining from A -> C and C back to A on a different field. If this is the case where two fields are in the "C" table that BOTH point to A, you would re-join to A as a second alias...
select
Want.*,
Maybe.*,
SecondA.*,
B.*
From
A as Want
LEFT JOIN C as Maybe
on Want.ID = Maybe.ID
JOIN A as SecondA
on Maybe.AID = SecondA.ID
JOIN B
on Maybe.ID = B.cID
So, this query is stating I want everything from Table A (alias Want -- left side/first table in the list) Regardless of there being a match in Table C (alias Maybe) where the ID keys match.
Notice the next joins going down from "C" back to the second instance of "A" and table B. I have those as just joins... So the relationship between the "Maybe" alias, and that of second instance of "A" and "B" are JOIN (required).
Hopefully this gives some better clarification on HOW it works.
Now, for your real-life query. If you can describe what you are looking for, and your sample table structures / result expections, listing that could offer more explicit solution to your needs.
Hope this will help
SELECT
A.*, B.*, C.*
FROM A
inner join C on(A.id = C.id)
inner join B on(B.cid = C.id)