Let's say I have these tables and values:
Table1
------------------------
ID | Value
------------------------
2 | asdf
4 | fdsa
5 | aaaa
Table2
------------------------
ID | Value
------------------------
2 | bbbb
4 | bbbb
5 | bbbb
I want to update all the values in Table2 using the values in Table1 with their respective ID's.
I know I can run this:
UPDATE Table2
SET Value = t1.Value
FROM Table2 t2
INNER JOIN Table1 t1 on t1.ID = t2.ID
But what can I do if Table1 and Table2 are actually select statements with criteria? How can I modify the SQL statement to take that into consideration?
This is how such update queries are generally done in Oracle. Oracle doesn't have an UPDATE FROM option:
UPDATE table2 t2
SET t2.value = ( SELECT t1.value FROM table1 t1
WHERE t1.ID = t2.ID )
WHERE EXISTS ( SELECT 1 FROM table1 t1
WHERE t1.ID = t2.ID );
The WHERE EXISTS clause will make sure that only the rows with a corresponding row in table1 are updated (otherwise every row in table2 will be updated; those without corresponding rows in table1 will be updated to NULL).
Related
I want to SELECT one record from table1 (WHERE t1.id = 1) and then JOIN table2 and table3 (t2.field2 and t3.field3) to table1 but ONLY if the values exists (IS NOT NULL).
So for example, if the value doesn't exist for t3.field3, the field3 column is not displayed for that table...
t1
id | field1
---------------
1 | f1val
2 | f1val
3 | f1val
t2
id(fk) | field2
-------------------
1 | f2val
2 | null
3 | null
t3
id(fk) | field3
-------------------
1 | null
2 | f3val
3 | f3val
the code I tried to do is this:
SELECT t1.id, t2.field1, t3.field3
FROM (
SELECT t1.id
FROM t1
WHERE t1.id = 1
)
LEFT JOIN t2 ON t2.id = t1.id AND t2.id is not null
LEFT JOIN t3 ON t2.id = t1.id AND t3.id is not null;
The joined table returned from the query above looks like this:
id | field2 | field3
----------------------------
1 | f1val | null
However, since field3 is null, I want it to return only the id and field2 like this:
id | field2
----------------
1 | f1val
Your help will be highly appreciated.
You could return one column, using coalesce():
SELECT t1.id, COALESCE(t2.field1, t3.field3) as field_2_3
FROM t1 LEFT JOIN
t2
ON t2.id = t1.id LEFT JOIN
t3
ON t3.id = t1.id
WHERE t1.id = 1;
However, you cannot sometimes return two columns and sometimes return 3 columns.
Notes:
The subquery on t1 is utterly unnecessary. You can just apply the filter in a single WHERE clause.
The comparisons for IS NOT NULL are unnecessary because they fail the JOIN condition anyway.
The last JOIN condition is presumably on t3.id = t1.id.
In the scenario where there are two tables, one column in the first has a nullable key to another table.
table1_id | table1_key | table2_id | table2_value
----------+------------+-----------+--------------
1 | 1 | 1 | 3
2 | | |
3 | 3 | 3 | 1
4 | 1 | 1 | 3
With a single efficient statement, I want to get all rows from table1 and data from table2 if they exist.
My current method does a union between two statements.
SELECT
table1.id as table1_id,
table1.fkey as table1_key,
table2.id as table2_id,
table2.value as table_value
FROM
table1,
table2
WHERE
table1.fkey = table2.id
UNION
SELECT
table1.id as table1_id,
null,
null,
null
FROM
table1,
table2
WHERE
table1.fkey = NOT IN (SELECT id FROM table2)
How can this be done more efficiently in a single select statement?
A left join would do the job,
SELECT
table1.id as table1_id,
table1.fkey as table1_key,
table2.id as table2_id,
table2.value as table_value
FROM table1
LEFT OUTER JOIN table2
ON table1.fkey = table2.id
You need a join between table1 and table2 on the foreign key relationship.
From your question, I understand that column fkey in table1 is a foreign key to column id in table2.
You want to retrieve rows from table1 even if there is no matching row in table2. Hence you need a left outer join
select t1.id as t1_id
,t1.fkey as t2_id
,t2.value as t2_value
from table1 t1
left outer join table2 t2
on t1.fkey = t2.id
I'm trying to find a simple solution for my SQL Server problem.
I have two tables look like this:
table1
--id
-- data
table2
--id
--table1_id
--value
I have some records like this:
Table1
+-----------------------+
| id | data |
+-----------------------+
| 1 | ? |
+-----------------------+
| 2 | ? |
+-----------------------+
Table2
+-----------------------+
|id | table1_id | value |
+-----------------------+
| 1 | 1 | 'a' |
+-----------------------+
| 2 | 1 | 'b' |
+-----------------------+
| 3 | 2 | 'a' |
+-----------------------+
Now I want to get table1 with all it's additional values where the relation to table2 has 'a' AND 'b' as values.
So I would get the id 1 of table1.
Currently I have an query like this:
SELECT t1.[id], t1.[data]
FROM [table1] t1,
(SELECT [id]
FROM [table1] t1
JOIN [table2] t2 ON t1.[id] = t2.[table1_id] AND t2.[Value] IN('a', 'b')
GROUP BY t1[id]
HAVING COUNT(t2.[Value]) = 2) x
WHERE t1.id = x.id
Has anyone an idea on how to achieve my goal in a simpler way?
One way uses exists:
select t1.*
from table1 t1
where exists (select 1
from table2 t2
where t2.table1_id = t1.id and t2.value = 'a'
) and
exists (select 1
from table2 t2
where t2.table1_id = t1.id and t2.value = 'b'
);
This can take advantage of an index on table2(table1_id, value).
You could also write:
select t1.*
from table1 t1
where (select count(distinct t2.value)
from table2 t2
where t2.table1_id = t1.id and t2.value in ('a', 'b')
) = 2 ;
This would probably also have very good performance with the index, if table2 doesn't have duplicates.
SELECT T1.[id], T1.[data]
FROM table1 AS T1
JOIN table2 AS T2
ON T1.[id]=T2.[table1_id]
JOIN table2 AS T3
ON T1.[id]=T3.[table1_id]
WHERE
T2.[Value] ='a'
AND T3.[Value] = 'b'
As Gordon Linoff suggested, exists clause usage works as well and could be performance efficient depending on the data you are playing with.
you have to do several steps to solve the problem:
established which records are related to table 1 and table 2 and which of these are of value (A or B) and eliminate the repeated ones with the group by(InfoRelationate )
validate that only those related to a and b were allowed by means of a count in the table above (ValidateAYB)
see what data meets the condition of table1 and table 2 and joined table 1
this query meets the conditions
with InfoRelationate as
(
select Table2.table1_id,value
from Table2 inner join
Table1 on Table2.table1_id=Table1.id and Table2.value IN('a', 'b')
group by Table2.table1_id,value
),
ValidateAYB as
(
select InfoRelationate.table1_id
from InfoRelationate
group by InfoRelationate.table1_id
having count (1)=2
)
select InfoRelationate.table1_id,InfoRelationate.value
from InfoRelationate
inner join ValidateAYB on InfoRelationate.table1_id=ValidateAYB.table1_id
union all
select id,data
from Table1
Example code
I need to update the column data based on matching record found in both Table.
I want to update the record of NAME column from TABLE2
Following are the tables
Table1
---------------
Id | Name | color
1 | abc | red
2 | def | green
3 | ghi | blue
Table2
---------------
Id | Name | color |fiedId
1 | abc | red | 1
2 | def | green | 1
3 | ghi | blue | 2
Here table1 ID column is the Foreign Key in table2 as fieldId.
So, I want to update all the record which fall under this condition
table1.id = table2.fieldId
Yet another option, using MERGE:
merge into table2 t2
using (select id, name from table1) x
on (t2.fieldid = x.id)
when matched then update set
t2.name = x.name;
Or, for setting the name to 'xxx':
merge into table2 t2
using (select id from table1) x
on (t2.fiedid = x.id)
when matched then update set
t2.name = 'xxx';
Sounds like you just want an update like this:
update table2 t2
set t2.name =
( select t1.name
from table1 t1
where t1.id = t2.fieldid )
Regarding the followup question:
what if i want to set Name = "xxx" for all matching rows?
update table2 t2
set t2.name = 'xxx'
where t2.fieldid in
( select t1.id from table1 t1 )
or this can be written as:
update table2 t2
set t2.name = 'xxx'
where exists
( select null from table1 t1
where t1.id = t2.fieldid )
My coworker, who is new to ANSI join syntax, recently wrote a query like this:
SELECT count(*)
FROM table1 t1
JOIN table2 t2 ON
(t1.col_a = t2.col_a)
JOIN table3 t3 ON
(t2.col_b = t3.col_b)
JOIN table3 t3 ON
(t3.col_c = t1.col_c);
Note that table3 is joined to both table1 and table2 on different columns, but the two JOIN clauses use the same table alias for table3.
The query runs, but I'm unsure of it's validity. Is this a valid way of writing this query?
I thought the join should be like this:
SELECT count(*)
FROM table1 t1
JOIN table2 t2 ON
(t1.col_a = t2.col_a)
JOIN table3 t3 ON
(t2.col_b = t3.col_b AND
t3.col_c = t1.col_c);
Are the two versions functionally identical? I don't really have enough data in our database yet to be sure.
Thanks.
The first query is a join of 4 tables, the second one is a join of 3 tables. So I don't expect that both queries return the same numbers of rows.
SELECT *
FROM table1 t1
JOIN table2 t2 ON
(t1.col_a = t2.col_a)
JOIN table3 t3 ON
(t2.col_b = t3.col_b)
JOIN table3 t3 ON
(t3.col_c = t1.col_c);
The alias t3 is only used in the ON clause. The alias t3 refers to the table before the ON keyword. I found this out by experimenting. So the pervious query is equvivalent to
SELECT *
FROM table1 t1
JOIN table2 t2 ON
(t1.col_a = t2.col_a)
JOIN table3 t3 ON
(t2.col_b = t3.col_b)
JOIN table3 t4 ON
(t4.col_c = t1.col_c);
and this can be transfotmed in a traditional join
SELECT *
FROM table1 t1,
table2 t2,
table3 t3,
table3 t4
where (t1.col_a = t2.col_a)
and (t2.col_b = t3.col_b)
and (t4.col_c = t1.col_c);
The second query is
SELECT *
FROM table1 t1
JOIN table2 t2 ON
(t1.col_a = t2.col_a)
JOIN table3 t3 ON
(t2.col_b = t3.col_b AND
t3.col_c = t1.col_c);
This can also transformed in a traditional join
SELECT *
FROM table1 t1,
table2 t2,
table3 t3
where (t1.col_a = t2.col_a)
and (t2.col_b = t3.col_b)
AND (t3.col_c = t1.col_c);
These queries seem to be different. To proof their difference we use the following example:
create table table1(
col_a number,
col_c number
);
create table table2(
col_a number,
col_b number
);
create table table3(
col_b number,
col_c number
);
insert into table1(col_a, col_c) values(1,3);
insert into table1(col_a, col_c) values(4,3);
insert into table2(col_a, col_b) values(1,2);
insert into table2(col_a, col_b) values(4,2);
insert into table3(col_b, col_c) values(2,3);
insert into table3(col_b, col_c) values(2,5);
insert into table3(col_b, col_c) values(7,9);
commit;
We get the following output
SELECT *
FROM table1 t1
JOIN table2 t2 ON
(t1.col_a = t2.col_a)
JOIN table3 t3 ON
(t2.col_b = t3.col_b)
JOIN table3 t3 ON
(t3.col_c = t1.col_c)
| COL_A | COL_C | COL_A | COL_B | COL_B | COL_C | COL_B | COL_C |
|-------|-------|-------|-------|-------|-------|-------|-------|
| 1 | 3 | 1 | 2 | 2 | 3 | 2 | 3 |
| 4 | 3 | 4 | 2 | 2 | 3 | 2 | 3 |
| 1 | 3 | 1 | 2 | 2 | 5 | 2 | 3 |
| 4 | 3 | 4 | 2 | 2 | 5 | 2 | 3 |
SELECT *
FROM table1 t1
JOIN table2 t2 ON
(t1.col_a = t2.col_a)
JOIN table3 t3 ON
(t2.col_b = t3.col_b AND
t3.col_c = t1.col_c)
| COL_A | COL_C | COL_A | COL_B | COL_B | COL_C |
|-------|-------|-------|-------|-------|-------|
| 4 | 3 | 4 | 2 | 2 | 3 |
| 1 | 3 | 1 | 2 | 2 | 3 |
The number of rows retrieved is different and so count(*) is different.
The usage of the aliases was surprising. at least for me.
The following query works because t1 in the where_clause references table2.
select *
from table1 t1 join table2 t1 on(1=1)
where t1.col_b<0;
The following query works because t1 in the where_clause references table1.
select *
from table1 t1 join table2 t1 on(1=1)
where t1.col_c<0;
The following query raises an error because both table1 and table2 contain a column col_a.
select *
from table1 t1 join table2 t1 on(1=1)
where t1.col_a<0;
The error thrown is
ORA-00918: column ambiguously defined
The following query works, the alias t1 refers to two different tables in the same where_clause.
select *
from table1 t1 join table2 t1 on(1=1)
where t1.col_b<0 and t1.col_c<0;
These and more examples can be found here: http://sqlfiddle.com/#!4/84feb/12
The smallest counter example
The smallest counter example is
table1
col_a col_c
1 2
table2
col_a col_b
1 3
table3
col_b col_c
3 5
6 2
Here the second query has an empty result set and the first query returns one row. It can be shown that the count(*) of the second query never exeeds the count(*)of the first query.
A more detailed explanation
This behaviour will became more clear if we analyze the following statement in detail.
SELECT t.col_b, t.col_c
FROM table1 t
JOIN table2 t ON
(t.col_b = t.col_c) ;
Here is the reduced syntax for this query in Backus–Naur form derived from the syntax descriptions in the SQL Language Reference of Oracle 12.2. Note that under each syntax diagram there is a link to the Backus–Naur form of this diagram, e.g Description of the illustration select.eps. "reduced" means that I left out all the possibilities that where not used, e,g. the select is defined as
select::=subquery [ for_update_clause ] ;
Our query does not use the optional for_update_clause, so I reduced the rule to
select::=subquery
The only exemption is the optional where-clause. I didn't remove it so that this reduced rules can be used to analyze the above query even if we add a where_clause.
These reduced rule will define only a subset of all possible select statements.
select::=subquery
subquery::=query_block
query_block::=SELECT select_list FROM join_clause [ where_clause ]
join_clause::=table_reference inner_cross_join_clause ...
table_reference::=query_table_expression t_alias query_table_expression::=table
inner_cross_join_clause::=JOIN table_reference ON condition
So our select statement is a query_block and the join_clause is of type
table_reference inner_cross_join_clause
where table_reference is table1 t and inner_cross_join_clause is JOIN table2 t ON (t.col_b = t.col_c). The ellipsis ... means that there could be additional inner_cross_join_clauses, but we do not need this here.
in the inner_cross_join_clause the alias t refers to table2. Only if these references cannot be satisfied the aliasmust be searched in an outer scope. So all the following expressions in the ONcondition are valid:
t.col_b = t.col_c
Here t.col_b is table2.col_b because t refers to the alias of its inner_cross_join_clause, t.col_c is table1.col_c. t of the inner_cross_join_clause (refering to table2) has no column col_c so the outer scope will be searched and an appropriate alias will be found.
If we have the clause
t.col_a = t.col_a
the alias can be found as alias defined in the inner_cross_join_clause to which this ON-condition belongs so t will be resolved to table2.
if the select list consists of
t.col_c, t.col_b, t.col_a
instead of * then the join_clause will be searched for an alias and t.col_c will be resolved to table1.col_c (table2 does not contain a column col_c), t.col_b will be resolved to table2.col_b (table1 does not contain a col_b) but t.col_a will raise the error
ORA-00918: column ambiguously defined
because for the select_list none of the aias definition has a precedenve over the other. If our query also has a where_clause then the aliases are resolved in the same way as if they are used in the select_list.
With more data, it will produce different results.
Your colleagues query is same as this.
select * from table3 where t3.col_b = 'XX'
union
select * from table3 where t3.col_c = 'YY'
or
select * from table3 where t3.col_b = 'XX' or t3.col_c = 'YY'
while your query is like this.
select * from table3 where t3.col_b ='XX' and t3.col_c='YY'
First one is like data where (xx or yy) while second one is data where ( xx and yy)