How can i join multiple tables not depending on LEFT or RIGHT join
example:
t1
id | date
-----------
NULL NULL
t2
id | value
------------
1 | bla
SELECT date,value
FROM t2 LEFT JOIN t1
ON t1.id = t2.id where t2.id = 1
-- select is ok
with right join same query return null values ...
SELECT `date`, `value` FROM t2 FULL OUTER JOIN t1 ON t1.id = t2.id
Edit: That will return all records, even if there is no match on those joining fields. Sorry, that will give you syntax error. See my edit down below.
If you only want matches, use inner join:
SELECT `date`, `value` FROM t2 INNER JOIN t1 ON t1.id = t2.id
Edit: There is no FULL OUTER JOIN in MySQL. You will have to simulate it with UNION and combine a LEFT and RIGHT JOIN:
SELECT `date`, `value` FROM t2 LEFT JOIN t1 ON t1.id = t2.id
UNION
SELECT `date`, `value` FROM t2 RIGHT JOIN t1 ON t1.id = t2.id
This will also return your NULL values, but they will not match, as NULL != NULL
You must use FULL OUTER JOIN synstax. In fact Left JOIN returns all results in Left column, whereas right join returns all results in right column. SO with full outer, both are returned.
SELECT date,value FROM t2 FULL OUTER JOIN t1 ON t1.id = t2.id where t2.id = 1
Related
SELECT t1.*, IFNULL(t2.profile_id, t3.profile_id) AS `profile_id`
FROM table1 AS t1
LEFT JOIN table2 AS t2
ON t1.id = t2.some_coulmn
LEFT JOIN table3 AS t3
ON t1.id = t3.some_coulmn
LEFT JOIN table4 AS t4
ON profile_id = t4.some_column
I'm trying to use an alias (profile_id) inside my join condition. It fails. Is there a way to do this?
Column aliases defined in th SELECT clause cannot be used in the join conditions. This is because the FROM clause is evaluated before the SELECT clause.
If I followed you correctly, you probably want:
SELECT t1.*, IFNULL(t2.profile_id, t3.profile_id) AS `profile_id`
FROM table1 AS t1
LEFT JOIN table2 AS t2
ON t1.id = t2.some_coulm
LEFT JOIN table3 AS t3
ON t1.id = t3.some_coulm
LEFT JOIN table4 AS t4
ON IFNULL(t2.profile_id, t3.profile_id) = t4.some_column
I have 2 tables and result as shown in the image below: MySQL DB
What would be best way to join the two tables so we get the result as shown above.
SELECT * FROM (SELECT id, desc FROM table2) as T1
LEFT JOIN (SELECT * FROM table1) as T2 ON T1.id = T2.id
I guess my SQL is not working.
You can use a LEFT JOIN with COALESCE:
SELECT t1.id, COALESCE(t2.desc, t1.desc) AS desc, t1.D1, t1.D2
FROM table1 as T1
LEFT JOIN table2 as T2 ON T1.id = T2.id
Use a left join with coalesce to prioritize table 2's values if they are present, but fallback on table 1's values if not.
select t1.id,
coalesce(t2.desc, t1.desc) as desc,
t1.d1, t1.d2
from table1 t1
left join table2 t2
on t2.id = t1.id
order by t1.id
You can use ifnull:
SELECT t1.id, ifnull(t2.desc, t1.desc) AS desc, t1.D1, t1.D2
FROM table1 as T1
LEFT JOIN table2 as T2 ON T1.id = T2.id
coalesce or case .. when is also possible. All together with the left join
Are there any reasons why UNIONs shouldn't be used in the WHERE clause of update subqueries? Or for that matter, even normal select subqueries?
Is there a better way such a query to eliminate the UNION?
Note that for my case, the UNION will result in a fairly small number of records.
UPDATE mytable
set mytable.bla='xxx'
WHERE id IN (
SELECT id
FROM t1
INNER JOIN t2 ON t2.t1_id=t1.id
LEFT OUTER JOIN t3 ON t3.t1_id=t2.id
WHERE t2.id IN (1,2,3) AND t3.id IS NULL
UNION
SELECT id FROM t4
INNER JOIN t5 ON t5.id=t4.t5_id
LEFT OUTER JOIN t6 ON t6.t5_id=t5.id
WHERE t5.parent_id IN (1,2,3) AND t6.id IS NULL
);
Switching it to a join:-
UPDATE mytable
INNER JOIN
(
SELECT id
FROM t1
INNER JOIN t2 ON t2.t1_id=t1.id
LEFT OUTER JOIN t3 ON t3.t1_id=t2.id
WHERE t2.id IN (1,2,3)
AND t3.id IS NULL
UNION
SELECT id
FROM t4
INNER JOIN t5 ON t5.id=t4.t5_id
LEFT OUTER JOIN t6 ON t6.t5_id=t5.id
WHERE t5.parent_id IN (1,2,3)
AND t6.id IS NULL
) sub0
ON mytable.id = sub0.id
SET mytable.bla='xxx'
Whats the right MySQL query with two LEFT JOINs between three tables?
SELECT COUNT(1) FROM TABLE1 WHERE T1_ID NOT IN (
SELECT T2.T2_ID FROM TABLE2 T2 LEFT JOIN
TABLE3 T3 ON T2.T2_ID=T3.T3_ID WHERE T3.T3_ID IS NULL )
Something like
SELECT COUNT(1) FROM TABLE1 T1 LEFT JOIN TABLE2 T2 ON T1.T1_ID=T2.T2_ID
LEFT JOIN TABLE3 T3 ON T2.T2_ID=T3.T3_ID WHERE T2.T2_ID IS NULL AND
T3.T3_ID IS NULL
For performance I would do something like
SELECT
COUNT(1)
FROM
TABLE1
LEFT JOIN (
SELECT
T2.T2_ID id
FROM
TABLE2 T2
LEFT JOIN TABLE3 T3 ON
T2.T2_ID=T3.T3_ID
WHERE
T3.T3_ID IS NULL
) t1 ON
t1.id = table1.t1_id
WHERE
t1 is null;
I have been using the following SQL:
SELECT DISTINCT NAME
FROM Events t1
LEFT JOIN UserHistory t2 ON t1.Name = t2.Event
WHERE t2.Event IS NULL
To select all rows from table 1 where table 2 is Null. This effectively filters out all my Table 1 data where Table 2 has data. However, I want to apply this only when a column in table 2 equals a certain value. Therefore I am looking to do a SELECT * FROM t2 WHERE t2.ID = 1 but am unsure how this fits into this query.
SELECT DISTINCT NAME
FROM Events t1
LEFT JOIN UserHistory t2 ON t1.Name = t2.Event and t2.certain_column = 1234
WHERE t2.Event IS NULL
Also you can try query with NOT EXISTS:
SELECT DISTINCT NAME
FROM Events t1
WHERE NOT EXISTS(SELECT * FROM UserHistory t2
WHERE t1.Name = t2.Event AND t2.ID = 1)
You need to add the predicate to the JOIN condition:
SELECT DISTINCT NAME
FROM Events t1
LEFT JOIN UserHistory t2 ON t1.Name = t2.Event AND t2.ID = 1
WHERE t2.Event IS NULL;
If you add it to the WHERE you effectively turn your outer join into an inner join, meaning no rows will be returned (since NULL = 1 evaluates to false)