I need to join MySQL tables.
but when I apply where clause on the second table, query return no record.
I want a query that returns all records of the 1st table and joins with 2nd table data on certain conditions and if the condition does not find records in the second table then it appends null in the result.
Please see the attached picture for detail.
enter image description here
SELECT *
FROM `Table1`
JOIN `Table2` ON `Table1`.`id1` = `Table2`.`id2`
WHERE `Table2`.`department_id` = 2;
Use a left join, and move the where criteria to the join condition:
SELECT *
FROM Table1 t1
LEFT JOIN Table2 t2 ON t1.id1 = t2.id2 AND t2.department_id = 2;
Related
I've encountered a problem on my wordpress website. I have multiple select mysql query from one page then decided to merge because it's in from same table. But the problem is when first selected table is null, all other row is affected and becomes null. And if the first select table is has a value, then the others selected rows will be display.
Here's my sample mysql query code:
SELECT u1.meta_value as name, u2.meta_value as birthday,
u3.meta_value as place
FROM wp_usermeta u1
LEFT JOIN wp_usermeta u2 ON u1.user_id = u2.user_id
LEFT JOIN wp_usermeta u3 ON u1.user_id = u3.user_id
WHERE u1.user_id = 1092
AND u1.meta_key = 'name' AND u2.meta_key = 'birthday' AND u3.meta_key = 'place'
you can use union and 2nd query as a right join below is sample code
The LEFT JOIN keyword returns all records from the left table
(table1), and the matched records from the right table (table2). The
result is NULL from the right side, if there is no match.
The UNION operator is used to combine the result-set of two or more
SELECT statements.
Each SELECT statement within UNION must have the same number of
columns
The columns must also have similar data types
The columns in each SELECT statement must also be in the same order
select t1.* from t1 left join t2 on t1.id=t2.id
union
select t2.* from t1 right join t2 on t1.id=t2.id
When I run the following query I get 2769 rows returned.
SELECT *
FROM table1 t1
LEFT JOIN table2 t2 ON t2.account_id = t1.account_id;
However, when I add the WHERE clause below, I get 692 Lines returned.
SELECT *
FROM table1 t1
LEFT JOIN table2 t2 ON t2.account_id = t1.account_id
WHERE t2.account_id IN (t1.account_id);
I thought that the condition established by my LEFT JOIN would the same condition established by my WHERE clause (i.e. that these two lines would effectively be redundant).
This is clearly not the case, but I cannot figure out why.
The left join returns all records of t1 and returns null for the columns of t2 for all records where the join could not be made.
But the where clause filters all data, no matter from which table. So when you filter on t2 in your where clause then all records where the join could not be made (and the t2.account_id is null) get excluded from the result since null != t1.account_id.
So basically your where clause turn your left join into an inner join.
How would I go about writing a query in MySQL on 3 different tables? Here's what I have so far:
SELECT distinct cool_id, example, table1.name
FROM tardis
INNER JOIN table1
ON table1.unique_id = table2.unique_id
AND table1.unique_id='12345'
AND table2.status='active'
Now let's say that there is a column called 'planets' that exists in a 3rd table. How would I add that to this query to select 'planets' in addition to matching the other conditions in my current query? Also, please advise if an INNER JOIN is not the best choice for this.
Just add another INNER JOIN clause:
SELECT columns
FROM table1
INNER JOIN table2 ON table1.col1 = table2.col2
INNER JOIN table3 ON table1.somecol = table3.othercol
The ON condition in the second join can refer to columns from table1, table2, or both.
This is my first Table.
Then the second one is
Now I am trying to do Left join like this
SELECT t1.StackID FROM t1 LEFT JOIN t2 on t1.StackID=t2.StackID
Output
I am confused here, Is this the correct output? Is not this supposed to return only the 5 rows which is present in left side table.
It's correct output. You are doing LEFT JOIN, so for every record in LEFT table DBMS will 'concatenate' the corresponding RIGHT table record(-s) (and NULL, if there's no corresponding record for the JOIN condition; also remember, that if there are more that 1 corresponding record - all will be joined - this issue is why you're getting not only 5 records from 1-st table).
The thing you're trying to achieve should be done by either DISTINCT modifier, i.e.
SELECT DISTINCT t1.StackID FROM t1 LEFT JOIN t2 ON t1.StackID=t2.StackID;
More about JOIN in SQL you can read here.
There is a simple example of left join:-
SELECT * FROM a JOIN b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d ON (d.key=a.key)
WHERE b.key=d.key;
These extra repeated values of StackId are coming from the join, just use DISTINCT to get those only 5 values:
SELECT DISTINCT t1.StackID
FROM t1
LEFT JOIN t2 on t1.StackID=t2.StackID;
SQL Fiddle Demo
Yes, this is the expected output.
To get the distinct IDs, use DISTINCT -
SELECT DISTINCT t1.StackID
FROM t1
LEFT JOIN t2
ON t1.StackID=t2.StackID
When you left joined your t2 table with t1 based on stackID, then the database will join every row of t2 with the every row of t1 which has the same stackID value. Since 1 appears in six rows in the t2 table as a stackID value, all of them are joined with the first row of t1. Similar thing is true for all other values.
To give yourself a little more understanding of how a left join works try this:
SELECT t1.StackID, t2.StackID FROM t1 LEFT JOIN t2 on t1.StackID=t2.StackID
basically a left join returns all the rows from the left table plus matching ones from the right.
I'm sure this is straight-forward, but how do I write a query in mysql that joins two tables and then returns only those records from the first table that don't match. I want it to be something like:
Select tid from table1 inner join table2 on table2.tid = table1.tid where table1.tid != table2.tid;
but this doesn't seem to make alot of sense!
You can use a left outer join to accomplish this:
select
t1.tid
from
table1 t1
left outer join table2 t2 on
t1.tid = t2.tid
where
t2.tid is null
What this does is it takes your first table (table1), joins it with your second table (table2), and fills in null for the table2 columns in any row in table1 that doesn't match a row in table2. Then, it filters that out by selecting only the table1 rows where no match could be found.
Alternatively, you can also use not exists:
select
t1.tid
from
table1 t1
where
not exists (select 1 from table2 t2 where t2.tid = t1.tid)
This performs a left semi join, and will essentially do the same thing that the left outer join does. Depending on your indexes, one may be faster than the other, but both are viable options. MySQL has some good documentation on optimizing the joins, so you should check that out..