I am new to postgresql (and SQL in general) and my biggest concern right now is optimization. Below I have a simple query. However I believe that it is just one query. However, it could also be the case that it is just 3 queries (the ones that exist new the union all). Or it might actually be 6 queries since there are actually 6 select statements.
with foo as (
select * from tableA
), bar as (
select * from tableB
), zeta as (
select * from tableC
)
select * from foo
union all
select * from bar
union all
select * from zeta
How many queries is the above query?
also...
The reason this question is important is because, I do not know whether I should write my query like the above or to simply just make all the queries separate.
select * from tableA; --query 1
select * from tableB; --query 2
select * from tableC; --query 3
Formally it is one complex query (as 'complex sentence'), which contains six 'simple' queries. You do not need these with statements though, as in fact they do nothing. Your first query should look like this:
select * from tableA
union all
select * from tableB
union all
select * from tableC;
In this case you'll obtain one set of rows as a result. If you execute
select * from tableA;
select * from tableB;
select * from tableC;
you will have three separate sets of rows.
Related
With a query like this (simplified for clarity):
SELECT 'East' AS name, *
FROM events
WHERE event_timestamp BETWEEN '2015-06-14 06:15:00' AND '2015-06-21 06:15:00'
UNION
SELECT 'West' AS name, *
FROM events
WHERE event_timestamp BETWEEN '2015-06-14 06:15:00' AND '2015-06-21 06:15:00'
UNION
SELECT 'Both' AS name, *
FROM events
WHERE event_timestamp BETWEEN '2015-06-14 06:15:00' AND '2015-06-21 06:15:00'
I want to customise the order of the resulting rows. Something like:
ORDER BY name='East', name='West', name='Both'
Or
ORDER BY
CASE
WHEN name='East' THEN 1
WHEN name='West' THEN 2
WHEN name='Both' THEN 3
ELSE 4
END;
However, Postgres complains with:
ERROR: invalid UNION/INTERSECT/EXCEPT ORDER BY clause
DETAIL: Only result column names can be used, not expressions or functions.
HINT: Add the expression/function to every SELECT, or move the UNION into a FROM clause.
Do I have any alternative?
Wrap it in a derived table (which is what "HINT: .... or move the UNION into a FROM clause" is suggesting)
select *
from (
... your union goes here ...
) t
order by
CASE
WHEN name='East' THEN 1
WHEN name='West' THEN 2
WHEN name='Both' THEN 3
ELSE 4
END;
I'd add an extra column showing the desired ordering, then use ordinal column positions in the ORDER BY, e.g.
SELECT 1, 'East' AS name, *
...
UNION ALL
SELECT 2, 'West' AS name, *
...
ORDER BY 1
Note that you probably also want UNION ALL since your added columns ensure that every set in the union must be distinct anyway.
By adding an extra column for ordering purpose, however it makes the UNION clause to work exactly as a UNION ALL (it does not eliminate duplicate rows from the result).
If I try to UNION (or INTERSECT or EXCEPT) a common table expression I get a syntax error near the UNION. If instead of using the CTE I put the query into the union directly, everything works as expected.
I can work around this but for some more complicated queries using CTEs makes things much more readable. I also just don't like not knowing why something is failing.
As an example, the following query works:
SELECT *
FROM
(
SELECT oid, route_group
FROM runs, gpspoints
WHERE gpspoints.oid = runs.start_point_oid
UNION
SELECT oid, route_group
FROM runs, gpspoints
WHERE gpspoints.oid = runs.end_point_oid
) AS allpoints
;
But this one fails with:
ERROR: syntax error at or near "UNION"
LINE 20: UNION
WITH
startpoints AS
(
SELECT oid, route_group
FROM runs, gpspoints
WHERE gpspoints.oid = runs.start_point_oid
),
endpoints AS
(
SELECT oid, route_group
FROM runs, gpspoints
WHERE gpspoints.oid = runs.end_point_oid
)
SELECT *
FROM
(
startpoints
UNION
endpoints
) AS allpoints
;
The data being UNIONed together is identical but one query fails and the other does not.
I'm running PostgreSQL 9.3 on Windows 7.
The problem is because CTEs are not direct text-substitutions and a UNION b is invalid SELECT syntax. The SELECT keyword is a mandatory part of the parsing and the syntax error is raised before the CTEs are even taken into account.
This is why
SELECT * FROM a
UNION
SELECT * FROM b
works; the syntax is valid, and then the CTEs (represented by a and b) are then used at the table-position (via with_query_name).
At least in SQL Server, I can easily do this - create two CTE's, and do a SELECT from each, combined with a UNION:
WITH FirstNames AS
(
SELECT DISTINCT FirstName FROM Person
), LastNames AS
(
SELECT DISTINCT LastName FROM Person
)
SELECT * FROM FirstNames
UNION
SELECT * FROM LastNames
Not sure if this works in Postgres, too - give it a try!
Currently I've a query as follows:
-- Query 1
SELECT
acc_code, acc_name, alias, LAmt, coalesce(LAmt,0) AS amt
FROM
(SELECT
acc_code, acc_name, alias,
(SELECT
(SUM(cr_amt)-SUM(dr_amt))
FROM
ledger_mcg l
WHERE
(l.acc_code LIKE a.acc_code + '.%' OR l.acc_code=a.acc_code)
AND
fy_id=1
AND
posted_date BETWEEN '2010-01-01' AND '2011-06-02') AS LAmt
FROM
acc_head_mcg AS a
WHERE
(acc_type='4')) AS T1
WHERE
coalesce(LAmt,0)<>0
Query 2 is same as Query 1 except that acc_type = '5' in Query 2. Query 2 always returns a resultset with a single row. Now, I need the union of the two queries i.e
Query 1
UNION
Query 2
only when the amt returned by Query 2 is less than 0. Else, I don't need a union but only the resulset from Query 1.
The best way I can think of is to create a parameterised scalar function. How best can I do this?
You could store the result of the first query into a temporary table, then, if the table wasn't empty, execute the other query.
IF OBJECT_ID('tempdb..#MultipleQueriesResults') IS NOT NULL
DROP TABLE #MultipleQueriesResults;
SELECT
acc_code, acc_name, alias, LAmt, coalesce(LAmt,0) AS amt
INTO #MultipleQueriesResults
FROM
(SELECT
acc_code, acc_name, alias,
(SELECT
(SUM(cr_amt)-SUM(dr_amt))
FROM
ledger_mcg l
WHERE
(l.acc_code LIKE a.acc_code + '.%' OR l.acc_code=a.acc_code)
AND
fy_id=1
AND
posted_date BETWEEN '2010-01-01' AND '2011-06-02') AS LAmt
FROM
acc_head_mcg AS a
WHERE
(acc_type='4')) AS T1
WHERE
coalesce(LAmt,0)<>0;
IF NOT EXISTS (SELECT * FROM #MultipleQueriesResults)
… /* run Query 2 */
having a sql e.g. something like the following resulting in some rows with one value.
I search a different sql than SELECT * FROM some_sql which results in one row with comma separated values.
WITH some_sql AS (
SELECT 1 FROM DUAL
UNION
SELECT 2 FROM DUAL
)
SELECT * FROM some_sql
this SQL results in the two rows with value 1 and 2.
I seach a SQl resulting in 1,2 without changing the code of 'some_sql'.
Consider http://halisway.blogspot.com/2006/08/oracle-groupconcat-updated-again.html
Sice you are on 11G you can use LISTAGG
WITH some_sql AS (
SELECT 1 x FROM DUAL
UNION
SELECT 2 x FROM DUAL
)
SELECT LISTAGG(x, ',') WITHIN GROUP(ORDER BY x) FROM some_sql
I'm making select with a union.
SELECT * FROM table_1
UNION
SELECT * FROM table_2
Is it possible to filter query results by column values?
Yes, you can enclose your entire union inside another select:
select * from (
select * from table_1 union select * from table_2) as t
where t.column = 'y'
You have to introduce the alias for the table ("as t"). Also, if the data from the tables is disjoint, you might want to consider switching to UNION ALL - UNION by itself works to eliminate duplicates in the result set. This is frequently not necessary.
A simple to read solution is to use a CTE (common table expression). This takes the form:
WITH foobar AS (
SELECT foo, bar FROM table_1
UNION
SELECT foo, bar FROM table_2
)
Then you can refer to the CTE in subsequent queries by name, as if it were a normal table:
SELECT foo,bar FROM foobar WHERE foo = 'value'
CTEs are quite powerful, I recommend further reading here
One tip that you will not find in that MS article is; if you require more than one CTE put a comma between the expression statements. eg:
WITH foo AS (
SELECT thing FROM place WHERE field = 'Value'
),
bar AS (
SELECT otherthing FROM otherplace WHERE otherfield = 'Other Value'
)
If you want to filter the query based on some criteria then you could do this -
Select * from table_1 where table_1.col1 = <some value>
UNION
Select * from table_2 where table_2.col1 = <some value>
But, I would say if you want to filter result to find the common values then you can use joins instead
Select * from table_1 inner join table_2 on table_1.col1 = table_2.col1