I want to update a table with values from another table, which not always exist. So I need to left join the other table. The only way I found is this:
UPDATE lessonentity update
SET title=a.test
FROM lessonentity l
LEFT JOIN (SELECT 'hoho1' test) a ON(true)
where l.lessonid=48552
AND update.lessonid=l.lessonid
My question: Is it possible to left-join another table, without inner-joining (where) the updating-table again?
Yes, but not using an explicit join. In your case, this is sufficient given that a has only one row:
UPDATE lessonentity le
SET title = a.test
FROM (SELECT 'hoho1' test) a
WHERE le.lessonid = 48552;
Normally, there would be an additional condition in the WHERE, connecting a and le, but that is not necessary in this case because the table has a single row.
Related
Is it possible in Postgres to have an optional join?
My use case is something like
select ...
from a
inner join b using (b_id)
where b.type in (...)
a is a very large reporting table. b is used to filter a, BUT the most common use case is that we will want all b.types, and therefore all the b records in the join. In other words, in most cases we don't want to filter by b at all, and would not need the join in that case, but the filtering optionality still needs to be there in cases when the user wants to filter by type.
So is it possible to invoke the join optionally, and save the join effort in cases when we just want all of a?
If not, what's my next best option? IF ... THEN or CTE with a union of separate queries?
If you don't need any of b's columns, there is no need to JOIN table b, You can filter by using EXISTS(SELECT .. FROM b WHERE ...).
If you want to conditionally exclude a part of the WHERE clause, you could use the following construct: (the ignore_b boolean will function as an on/off switch)
-- $ignore_b is a Boolean flag
-- when True, the optimiser will ignore the exists(...)
SELECT ...
FROM a
WHERE ( $ignore_b OR EXISTS (
SELECT *
FROM b
WHERE b.b_id = a.some_id
AND b.type in (1,2,3,4,5)
)
);
In our example, you are still filtering based on b, based on whether a row with that b_id exists in b in the first place.
Postgresql will remove unneeded joins under very specific circumstances. You write the join as a left join, so that no rows of A can be removed due to the absence of corresponding rows in B. The column B.b_id is a declared unique or primary key, so that no rows of A can be duplicated due to duplicate matches in B. And of course, no column of B can referenced in the query (except the reference to the key column in the left join condition).
In those cases, you can just always write the LEFT JOIN, and PostgreSQL will figure out that it can skip it.
You can argue that if you have a declared foreign key constraint on the join condition, then you shouldn't need the JOIN to be a LEFT JOIN in order to implement this optimization. I think that that argument is correct, but PostgreSQL does not implement it that way.
I would just do it programatically. If you are already programmatically adding references to B in the WHERE clause, you should be able to do it for the join as well.
Context
Goal
I want to add a new column in a table.
I also want all existing columns to have a default value, depending on another column's value in another table.
What I have
I have created a new changelog file :
Issue
When I do a clean/install, I get the following error :
What have I done wrong ?
Can you try changing your SQL update statement with the below one and check if it works:
update assignment set location = 'SITE_TSN' from requirement where (requirement.id = assignment.requirement_id and requirement.location = 'INTRAMUROS');
In postgres when we want to update data in a table based on values in another table, we can use PostgreSQL UPDATE JOIN syntax :
UPDATE t1
SET t1.c1 = new_value
FROM t2
WHERE t1.c2 = t2.c2;
To join to another table in the UPDATE statement, you specify the joined table in the FROM clause and provide the join condition in the WHERE clause. The FROM clause must appear immediately after the SET clause.
For each row of table t1, the UPDATE statement examines every row of table t2. If the value in the c2 column of table t1 equals the value in the c2 column of table t2, the UPDATE statement updates the value in the c1 column of the table t1 the new value (new_value).
For a detailed example of this JOIN syntax, please visit this link.
Is it possible to add a new column to an existing table from another table using insert or update in conjunction with full outer join .
In my main table i am missing some records in one column in the other table i have all those records i want to take the full record set into the maintable table. Something like this;
UPDATE maintable
SET all_records= othertable.records
FROM
FULL JOIN othertable on maintable.col = othertable.records;
Where maintable.col has same id a othertable.records.
I know i could simply join the tables but i have a lot of comments in the maintable i don't want to have to copy paste back in if possible. As i understand using where is equivalent of a left join so won't show me what i'm missing
EDIT:
What i want is effectively a new maintable.col with all the records i can then pare down based on presence of records in other cols from other tables
Try this:
UPDATE maintable
SET all_records = o.records
FROM othertable o
WHERE maintable.col = o.records;
This is the general syntax to use in postgres when updating via a join.
HTH
EDIT
BTW you will need to change this - I used your example, but you are updating the maintable with the column used for the join! Your set needs to be something like SET missingcol = o.extracol
AMENDED GENERALISED ANSWER (following off-line chat)
To take a simplified example, suppose that you have two tables maintable and subtable, each with the same columns, but where the subtable has extra records. For both tables id is the primary key. To fill maintable with the missing records, for pre 9.5 versions of Postgres you must use the following syntax:
INSERT INTO maintable (SELECT * FROM subtable s WHERE NOT EXISTS
(SELECT 1 FROM maintable m WHERE m.id = s.id));
Since 9.5 there is a (preferred) alternative:
INSERT INTO maintable (SELECT * FROM subtable) ON CONFLICT DO NOTHING;
This is preferred because (apart from being simpler) it avoids the situation that has been known to arise in the former, where a race condition is created between the INSERT and the sub-SELECT.
Obviously when the columns are different, you need to specify in the INSERT statement which columns are inserted from which. Something like:
INSERT INTO maintable (id, ColA, ColB)
(SELECT id, ColE, ColG FROM subtable ....)
Similarly the common field might not be id in both tables. However, the simplified example should be enough to point you in the right direction.
I'm trying to build a request to get the data from a table, but some of those columns have foreign keys I would like to replace by the associated keyword in one request.
Basically there's
table A with column 1:PKA-ID and column 2:name.
table B with column 1:PKB-ID, column 2:FKA-ID, column 3:amount.
I want to get all the lines in table B but with all foreign keys replaced by the associated names in table A.
I started building a request with a subrequest + alias to get that, but ofc I have more than one result per subrequest, yet I can't find a way to link that subrequest to the ID of table B [might be exhausted, dumb or both] from the main request. I did something like that:
SELECT (SELECT "NAME" FROM A JOIN B ON ID = FKA-ID) AS name, amount FROM TABLEB;
it feels so simple of a request yet...
You don't need a join in the subselect.
SELECT pkb_id,
(SELECT name FROM a WHERE a.pka_id = b.fka_id),
amount
FROM b;
(See it live in SQL Fiddle).
The subselect query runs for each and every row of its parent select and has the parent row available from the context.
You can also use a simple join.
SELECT b.pkb_id, a.name, b.amount
FROM b, a
WHERE a.pka_id = b.fka_id;
Note that the join version puts less restrictions on the PostgreSQL query optimizer so in some cases the join version might work faster. (For example, in PostgreSQL 9.6 the join might utilize multiple CPU units, cf. Parallel Query).
I have this query here which returns an error because of too many rows returned:
UPDATE tmp_rsl2 SET comm_percent=( SELECT c2.comm_percent
FROM tmp_rsl2 t1
INNER JOIN gn_salesperson g1 ON t1.sales_person=g1.sales_person
INNER JOIN comm_schema c1 ON g1.comm_schema=c1.comm_schema
INNER JOIN comm_schema_dt c2 ON c1.comm_schema_id=c2.comm_schema_id AND (t1.balance_amount::numeric <= (COALESCE(c2.value_amount,0)) );`
Basically for each row of the comm_percent column, I want to update all of them using the subquery SELECT statement. I imagine using a FOR loop or something but I'd like to hear ideas or to know a proper way to do this.
The error TOO_MANY_ROWS is about assigning a value to a variable, that can only take '1' (one) value, whereas the SELECT query is returning more than one.
Without a reference schema, its difficult to give an SQL that'd work (not to say that the issue lies with the Schema), but you need to ensure that the value assigned to comm_percent from the SELECT statement returns only 1 row. A very blind attempt at how it 'might' work in your case (given below), but again without knowing the schema its difficult to gauge whether it'd work.
UPDATE tmp_rsl2
SET comm_percent = c2.comm_percent
FROM gn_salesperson g1 ON
INNER JOIN comm_schema c1 ON g1.comm_schema = c1.comm_schema
INNER JOIN comm_schema_dt c2 ON c1.comm_schema_id = c2.comm_schema_id
AND (tmp_rsl2.balance_amount::NUMERIC <= (COALESCE(c2.value_amount, 0)))
WHERE tmp_rsl2.sales_person = g1.sales_person
UPDATE
As per below comments, have given an unrelated SQLFiddle example that should give an idea of how to perform an UPDATE of all rows of a table looking up corresponding values from another table.