there is three table
table a
table b
table c
and my query is :
select a.title,a.id,b.title
from table a
case when a.type=1 then
inner join table a.id=tableb.id end
case when a.type=2 then inner join table a.id=table c.id
But this query doesnt work.Can somebody helpe the right way to fetch or execute this type of query
You cannot use case in the from clause. To achieve this you could use UNION ALL. For instance:
select a.title,a.id,b.title
from table a inner join table b on a.id=b.id
where a.type=1
UNION ALL
select a.title,a.id,c.title
from table a inner join table c on a.id=c.id
where a.type=2
You cannot do something like "if this is 1 then join other table than if it is 2", you must join both and select accordingly:
SELECT
a.title,
a.id,
IF (tableb.title IS NOT NULL, tableb.title, tablec.title),
CASE a.type
WHEN 1 THEN tableb.id
WHEN 2 THEN tablec.id
END
FROM table a
LEFT JOIN tableb ON tablea.id = tableb.id
LEFT JOIN tablec ON tablea.id = tablec.id
Related
I would like to create equivalent MySQL query using LEFT OUTER JOIN to WHERE EXISTS. I am following this question:
Are the SQL concepts LEFT OUTER JOIN and WHERE NOT EXISTS basically the same?
This is the original query:
SELECT *
FROM tableA
JOIN tableB ON tableA.tableA_id = tableB.tableB_id
JOIN tableC ON tableC.tableC_id = tableB.tableB_id
WHERE NOT EXISTS (
SELECT 1
FROM tableD
WHERE tableA.employee_id = tableD.employee_id AND tableC.tableC_datum = DATE(tableD.tableD_od_datetime)
)
But this query return different values:
SELECT *
FROM tableA
JOIN tableB ON tableA.tableA_id = tableB.tableB_id
JOIN tableC ON tableC.tableC_id = tableB.tableB_id
LEFT OUTER JOIN tableD ON tableA.employee_id = tableD.employee_id AND tableC.tableC_datum = DATE(tableD.tableD_od_datetime)
WHERE tableD.employee_id IS NULL AND DATE(tableD.tableD_od_datetime) IS NULL
Why are these two outputs not equivalent, please?
The not exists and left join ... rgt.col is null approaches are identical. The left join however will contain columns from the unwanted table so just be specific with the select clause:
SELECT table_a.*, table_b.*, table_c.*
FROM table_a
JOIN table_b ...
JOIN table_c ...
LEFT JOIN table_d ...
I would rather avoid * at all and explicitly list exactly those columns that I need.
I have a SQL statement that does left join with a table:
select a.id, b.col1 from tableA a
left join (select col1 from tableB where date = 'xxx') b on a.id = b.id
For some application constraint (I need to use Spring JPQL query that does not permit subquery), I need to "flatten" this query to remove the subquery without changing the meaning of the query: I want to enrich tableA with a subset of tableB.
I have tried a few queries such as:
select a.id, b.col1 from tableA a
left join tableB b on a.id = b.id
where (date = 'xxx' or date is null)
But that gave me different set of answer from previous query.
How do I remove this subquery?
It can be done in multiple different ways - using cte, using joins
Using join it can be implemented as -
select a.id, b.col1 from tableA a left join tableB b on a.id = b.id and b.date = 'xxx'
using CTE it can be implemented as -
with t as
(
select col1, id from tableB where date = 'xxx'
)
select a.id, b.col1 from tableA a
left join t on a.id = t.id
I'd like to bring in just the most recent date row along with other fields from Table B that would include all values for multiple columns in Table A, even if there is no value in Table B.
The following query works, but only returns values that exist in both Table A AND in Table B. How do I include all of A and the most recent date row of B?
Many Thanks!
SELECT A.person_id,
A.second_field,
B.create_timestamp,
B.second_field
FROM (
SELECT B.person_id, max(B.create_timestamp) as create_timestamp
FROM Table_B B
GROUP BY B.person_id
) BMaxDate
LEFT JOIN Table_B B
ON BMaxDate.person_id = B.person_id AND
BMaxDate.create_timestamp = B.create_timestamp
LEFT JOIN Table_A A ON BMaxDate.person_id = A.person_id
You have to use Table_A as the first table in the LEFT JOIN operation chain:
SELECT A.person_id,
A.second_field,
B.create_timestamp,
B.second_field
FROM Table_A AS A
LEFT JOIN (
SELECT B.person_id, max(B.create_timestamp) as create_timestamp
FROM Table_B B
GROUP BY B.person_id
) AS BMaxDate ON BMaxDate.person_id = A.person_id
LEFT JOIN Table_B AS B
ON BMaxDate.person_id = B.person_id AND
BMaxDate.create_timestamp = B.create_timestamp
If you want all from table_A and use a left join you have to put table_A first.
And for Sql Server you can use the with clause for more readable sub query. Mysql doesn't support the with clause.
With(
SELECT B.person_id, max(B.create_timestamp) as create_timestamp
FROM Table_B B
GROUP BY B.person_id
) AS BMaxDate
SELECT A.person_id,
A.second_field,
B.create_timestamp,
B.second_field
FROM Table_A AS A
LEFT JOIN BMaxDate ON BMaxDate.person_id = A.person_id
LEFT JOIN Table_B AS B ON BMaxDate.person_id = B.person_id
AND BMaxDate.create_timestamp = B.create_timestamp
How get a distinct value form more than one table (inner join query).
Eg,
select a.id,b.name,c.address
from table1 a
inner join table2 b on (a.id = b.row_id)
inner join table3 c on (a.id = c.ticket_id)
where c.status = 'open';
Here the scenario is for example, two rows contain the same a.id value so how to get the distinct value from a.id.
Somebody help me that how to get?
just add Distinct ...
select DISTINCT a.id,b.name,c.address
from table1 a
inner join table2 b on (a.id = b.row_id)
inner join table3 c on (a.id = c.ticket_id)
where c.status = 'open';
i think this is works fine..
if you need only one record distinct then it should be like this...
SELECT DISTINCT(cat_id) FROM PRODUCTS WHERE brand_id = 'sony'
If inner join requires that a row exists, what's the opposite of it without having to do a sub query of NOT EXISTS?
I replaced
AND NOT EXISTS (
SELECT
*
FROM topic_read_assoc
WHERE topic_id = topic.id
AND member_id = ".$this->tru->application->currentMember->getId()."
)
with
OUTER JOIN topic_read_assoc ON (
topic_read_assoc.topic_id = topic.id AND
member_id = member_id = ".$this->tru->application->currentMember->getId()."
)
and it's not producing the same results as the first query (which works)
OUTER JOIN with a WHERE field IS NULL
Example:
SELECT A.name FROM A INNER JOIN B on A.id = B.id
Select those names in A whose id fields exist in B
Opposite:
SELECT A.name FROM A OUTER JOIN B on A.id = B.id WHERE B.id IS NULL
Select those names in A whose id fields do not exist in B
i think select on outer join is slow, because dbms left join first,then right join and delete the repeated rows.So I suggest you to select on the left join,then right join,make a intersect.It is better not operate on any join,because the view doesnt have index.