postgres: Querying across rows in multiple tables? - sql

Is there a way to apply a single query across the concatenated rows of multiple tables? I have several tables storing metrics data, where data is initially collected in 10 second intervals, but is periodically rolled up into 1 minute intervals in another table and ultimately into 10 minute intervals in a third table.
That is, I want to be able to do something like:
SELECT value FROM table1 + table2 + table3
WHERE age(current_timestamp, time) > '2 days'
AND metrics_name = 'foo';
I don't think a JOIN operation is the right solution here. It looks like I can get the result I want using the UNION operator, which would look something like:
SELECT value from TABLE1 ...
UNION SELECT value FROM table2 ...
UNION SELECT value from TABLE3 ...
Where ... is the contents of my WHERE clause (etc), but this gets messy very quickly if the individual SELECT queries are complicated (and fails the "don't repeat yourself" mantra). Is there a different way of cracking this particular nut?


Use a nested query with the UNION in the nested quer. And I highly recommend you use UNION ALL unless you want to eliminate duplicates, which you won't in this case, so use UNION ALL.
SELECT
value
FROM (
SELECT value, age, metrics_name FROM Table1
UNION ALL SELECT value age, metrics_name FROM Table2
UNION ALL SELECT value age, metrics_name FROM Table3
) AS All_Metrics
WHERE
All_Metrics.age(current_timestamp, time) > '2 days'
AND All_Metrics.metrics_name = 'foo';

Related

Join two SQL Server tables [duplicate]

This question already has answers here:
Combine two tables for one output
(2 answers)
Closed 8 years ago.
I have two tables now I need a select or join command in SQL to have the third table just like image below
My two tables are like this:
I only know a simple things about join command in SQL, should I use join or something else?
I do not want have the third table in my database, I want that for a short time (something like virtual table). Please help !

You are actually looking for UNION or UNION ALL.
First of all, there is no condition on which to JOIN tables (review your documentation on JOIN) and JOIN is used for retrieving information about one logical element, let's say Event in your case, which has details stored in more tables.
Secondly, JOIN will make one result set with all of the columns of your two tables, when actually you are not trying to get all columns, but all rows.
For this you will have to use UNION or UNION ALL like this:
SELECT
EventID,
ID,
EventName,
Date,
Pic,
Privacy
FROM Table1
UNION ALL
SELECT
PLID AS EventID,
ID AS ID,
PlaceName AS EventName,
Date AS Date,
NULL AS Pic,
NULL AS Privacy
FROM Table2
In order to sort the result you get from the result set returned by the queries above you will need to wrap your above SELECT statements with another SELECT and use a WHERE clause at that level, like below:
SELECT *
FROM (SELECT
EventID,
ID,
EventName,
Date,
Pic,
Privacy
FROM Table1
UNION ALL
SELECT
PLID AS EventID,
ID AS ID,
PlaceName AS EventName,
Date AS Date,
NULL AS Pic,
NULL AS Privacy
FROM Table2) AS Result
WHERE Date > '2014-05-26'

What you're looking to do is a UNION or UNION ALL, not a join. See: http://www.w3schools.com/sql/sql_union.asp
UNION combines two tables without connecting their content. Your example shows all 4 records from the original tables unmodified.
A JOIN solution links the two tables. It's very common and you will probably use it if you're building a relational database, but it won't give you the example result.
Since the two tables don't have identical # of columns, you have to help it out here:
SELECT EventID, EventName, Date, Pic, privacy FROM [table 1]
UNION ALL
SELECT PLID, PlaceName, Date, null, null FROM [table 2]

You want to have one table from two different tables. So you need unified result set from each by renaming column in SELECT statement:
SELECT `EventID` AS `ObjectID`, `EventName` AS `ObjectName`, .... FROM table_1 ...
similary with table_2
Then combine to one result set:
SELECT `ID` AS `ObjectID`, `EventName` AS `ObjectName`, .... FROM table_1 ...
UNION
SELECT `PlaceID` AS `ObjectID`, `PlaceName` AS `ObjectName`, .... FROM table_2 ...

My mistake, I didn't take the time to examine the pictures fully. you would have to use Union since you want to return what is in both tables.

Oracle Query on 24 tables with same columns

I have 24 tables table1, table2, table3 ... with same columns to keep track of customers data on hourly basis and one rate table which rate is applied for a specific hour, rateId is a foregin key in all the 24 other tables, i need a dynamic query to fetch data from those tables on date and time basis. Can any one provide an example or guide me for that query.

You should not store the same data in 24 different tables. Partitioning (mentioned in a comment) is a very good solution when you have lots and lots of data and want to split it for performance reasons.
In any case, one way to structure your query is:
select t.*
from ((select * from table1) union all
(select * from table2) union all
. . .
(select * from table24)
) t
where <whatever you want>
You can then join this to whatever other tables you like (using rateId, for instance), filter on the fields, or whatever.
If you need to know the table where something came from, then you can get this as well:
select t.*
from ((select t.*, 1 as which from table1 t) union all
(select t.*, 2 as which from table2 t) union all
. . .
(select t.*, 24 as which from table24 t)
) t
where <whatever you want>
Note: I am using * here because the OP explicitly states that the tables have the same format. Even so, it is probably a good idea to list all the columns in each subquery.
EDIT:
As Bill suggests in the comment, you might want to turn this into a view. That way, you can write lots of queries on the tables, without worrying about the detailed tables. (And, better yet, you can fix the data structure by combining the tables, then change the view, and existing queries will work).

Multiple SQL SELECT

I've 10 tables with a lot of records. All tables have "Date" column. I want extract all data from tables for date.
I can do 10 queries SELECT * FROM Table1 WHERE Date=dd/MM/yyyy, ect...but I want to do only a query with "multiple selection". How can I do this?
I'm not so skilled with SQL language.
EDIT: I'm working with Microsoft Access and also MySQL (for two different desktop application, but same problem).
Tables have different fields (just Date all in common), so It's not good the use of UNION.

SELECT *
FROM table1,
table2
WHERE table1.date = 'somedate'
AND table2.date = 'somedate'

Take a look at the UNION operator for including data from multiple SELECT statements.

Your question is not so clear. Based one what i understood, you can use Union SQL statement to combine your queries and make them as a single query. But if you want to query for different dates in different tables, you can use only use multiple queries .

If I right understand your question, and you want to get data from tables where
all 10 tables contains the same list of fields, you can use
SELECT * FROM Table1 WHERE Date=dd/MM/yyyy
UNION ALL
SELECT * FROM Table2 WHERE Date=dd/MM/yyyy
UNION ALL
...
UNION ALL
SELECT * FROM Table10 WHERE Date=dd/MM/yyyy
UNION ALL
If fields are different, you need to add the fields you want to get in result set:
SELECT field1, field2 FROM Table1 WHERE Date=dd/MM/yyyy
UNION ALL
SELECT field1, field2 FROM Table2 WHERE Date=dd/MM/yyyy
UNION ALL
...
UNION ALL
SELECT field1, field2 FROM Table10 WHERE Date=dd/MM/yyyy
UNION ALL

Be careful of performance issues when using Union. -- Be sure to run some tests comparing query times of the two approaches.
Depending on your database software, you could also use a stored procedure.
And also check that the date column is indexed in each of the tables.

distinct values from multiple fields within one table ORACLE SQL

How can I get distinct values from multiple fields within one table with just one request.
Option 1
SELECT WM_CONCAT(DISTINCT(FIELD1)) FIELD1S,WM_CONCAT(DISTINCT(FIELD2)) FIELD2S,..FIELD10S
FROM TABLE;
WM_CONCAT is LIMITED
Option 2
select DISTINCT(FIELD1) FIELDVALUE, 'FIELD1' FIELDNAME
FROM TABLE
UNION
select DISTINCT(FIELD2) FIELDVALUE, 'FIELD2' FIELDNAME
FROM TABLE
... FIELD 10
is just too slow

if you were scanning a small range in the data (not full scanning the whole table) you could use WITH to optimise your query
e.g:
WITH a AS
(SELECT field1,field2,field3..... FROM TABLE WHERE condition)
SELECT field1 FROM a
UNION
SELECT field2 FROM a
UNION
SELECT field3 FROM a
.....etc

For my problem, I had
WL1 ... WL2 ... correlation
A B 0.8
B A 0.8
A C 0.9
C A 0.9
how to eliminate the symmetry from this table?
select WL1, WL2,correlation from
table
where least(WL1,WL2)||greatest(WL1,WL2) = WL1||WL2
order by WL1
this gives
WL1 ... WL2 ... correlation
A B 0.8
A C 0.9
:)

The best option in the SQL is the UNION, though you may be able to save some performance by taking out the distinct keywords:
select FIELD1 FROM TABLE
UNION
select FIELD2 FROM TABLE
UNION provides the unique set from two tables, so distinct is redundant in this case. There simply isn't any way to write this query differently to make it perform faster. There's no magic formula that makes searching 200,000+ rows faster. It's got to search every row of the table twice and sort for uniqueness, which is exactly what UNION will do.
The only way you can make it faster is to create separate indexes on the two fields (maybe) or pare down the set of data that you're searching across.
Alternatively, if you're doing this a lot and adding new fields rarely, you could use a materialized view to store the result and only refresh it periodically.
Incidentally, your second query doesn't appear to do what you want it to. Distinct always applies to all of the columns in the select section, so your constants with the field names will cause the query to always return separate rows for the two columns.
I've come up with another method that, experimentally, seems to be a little faster. In affect, this allows us to trade one full-table scan for a Cartesian join. In most cases, I would still opt to use the union as it's much more obvious what the query is doing.
SELECT DISTINCT CASE lvl WHEN 1 THEN field1 ELSE field2 END
FROM table
CROSS JOIN (SELECT LEVEL lvl
FROM DUAL
CONNECT BY LEVEL <= 2);
It's also worthwhile to add that I tested both queries on a table without useful indexes containing 800,000 rows and it took roughly 45 seconds (returning 145,000 rows). However, most of that time was spent actually fetching the records, not running the query (the query took 3-7 seconds). If you're getting a sizable number of rows back, it may simply be the number of rows that is causing the performance issue you're seeing.

When you get distinct values from multiple columns, then it won't return a data table. If you think following data
Column A Column B
10 50
30 50
10 50
when you get the distinct it will be 2 rows from first column and 1 rows from 2nd column. It simply won't work.

And something like this?
SELECT 'FIELD1',FIELD1, 'FIELD2',FIELD2,...
FROM TABLE
GROUP BY FIELD1,FIELD2,...

What is the difference between UNION and UNION ALL?

What is the difference between UNION and UNION ALL?

UNION removes duplicate records (where all columns in the results are the same), UNION ALL does not.
There is a performance hit when using UNION instead of UNION ALL, since the database server must do additional work to remove the duplicate rows, but usually you do not want the duplicates (especially when developing reports).
To identify duplicates, records must be comparable types as well as compatible types. This will depend on the SQL system. For example the system may truncate all long text fields to make short text fields for comparison (MS Jet), or may refuse to compare binary fields (ORACLE)
UNION Example:
SELECT 'foo' AS bar UNION SELECT 'foo' AS bar
Result:
+-----+
| bar |
+-----+
| foo |
+-----+
1 row in set (0.00 sec)
UNION ALL example:
SELECT 'foo' AS bar UNION ALL SELECT 'foo' AS bar
Result:
+-----+
| bar |
+-----+
| foo |
| foo |
+-----+
2 rows in set (0.00 sec)

Both UNION and UNION ALL concatenate the result of two different SQLs. They differ in the way they handle duplicates.
UNION performs a DISTINCT on the result set, eliminating any duplicate rows.
UNION ALL does not remove duplicates, and it therefore faster than UNION.
Note: While using this commands all selected columns need to be of the same data type.
Example: If we have two tables, 1) Employee and 2) Customer
Employee table data:
Customer table data:
UNION Example (It removes all duplicate records):
UNION ALL Example (It just concatenate records, not eliminate duplicates, so it is faster than UNION):

UNION removes duplicates, whereas UNION ALL does not.
In order to remove duplicates the result set must be sorted, and this may have an impact on the performance of the UNION, depending on the volume of data being sorted, and the settings of various RDBMS parameters ( For Oracle PGA_AGGREGATE_TARGET with WORKAREA_SIZE_POLICY=AUTO or SORT_AREA_SIZE and SOR_AREA_RETAINED_SIZE if WORKAREA_SIZE_POLICY=MANUAL ).
Basically, the sort is faster if it can be carried out in memory, but the same caveat about the volume of data applies.
Of course, if you need data returned without duplicates then you must use UNION, depending on the source of your data.
I would have commented on the first post to qualify the "is much less performant" comment, but have insufficient reputation (points) to do so.

In ORACLE: UNION does not support BLOB (or CLOB) column types, UNION ALL does.

The basic difference between UNION and UNION ALL is union operation eliminates the duplicated rows from the result set but union all returns all rows after joining.
from http://zengin.wordpress.com/2007/07/31/union-vs-union-all/

UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table.
A UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.

You can avoid duplicates and still run much faster than UNION DISTINCT (which is actually same as UNION) by running query like this:
SELECT * FROM mytable WHERE a=X UNION ALL SELECT * FROM mytable WHERE b=Y AND a!=X
Notice the AND a!=X part. This is much faster then UNION.

Just to add my two cents to the discussion here: one could understand the UNION operator as a pure, SET-oriented UNION - e.g. set A={2,4,6,8}, set B={1,2,3,4}, A UNION B = {1,2,3,4,6,8}
When dealing with sets, you would not want numbers 2 and 4 appearing twice, as an element either is or is not in a set.
In the world of SQL, though, you might want to see all the elements from the two sets together in one "bag" {2,4,6,8,1,2,3,4}. And for this purpose T-SQL offers the operator UNION ALL.

UNION - results in distinct records while
UNION ALL - results in all the records including duplicates.
Both are blocking operators and hence I personally prefer using JOINS over Blocking Operators(UNION, INTERSECT, UNION ALL etc. ) anytime.
To illustrate why Union operation performs poorly in comparison to Union All checkout the following example.
CREATE TABLE #T1 (data VARCHAR(10))
INSERT INTO #T1
SELECT 'abc'
UNION ALL
SELECT 'bcd'
UNION ALL
SELECT 'cde'
UNION ALL
SELECT 'def'
UNION ALL
SELECT 'efg'
CREATE TABLE #T2 (data VARCHAR(10))
INSERT INTO #T2
SELECT 'abc'
UNION ALL
SELECT 'cde'
UNION ALL
SELECT 'efg'
Following are results of UNION ALL and UNION operations.
A UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.
Using UNION results in Distinct Sort operations in the Execution Plan. Proof to prove this statement is shown below:

Not sure that it matters which database
UNION and UNION ALL should work on all SQL Servers.
You should avoid of unnecessary UNIONs they are huge performance leak. As a rule of thumb use UNION ALL if you are not sure which to use.

(From Microsoft SQL Server Book Online)
UNION [ALL]
Specifies that multiple result sets are to be combined and returned as a single result set.
ALL
Incorporates all rows into the results. This includes duplicates. If not specified, duplicate rows are removed.
UNION will take too long as a duplicate rows finding like DISTINCT is applied on the results.
SELECT * FROM Table1
UNION
SELECT * FROM Table2
is equivalent of:
SELECT DISTINCT * FROM (
SELECT * FROM Table1
UNION ALL
SELECT * FROM Table2) DT
A side effect of applying DISTINCT over results is a sorting operation on results.
UNION ALL results will be shown as arbitrary order on results But UNION results will be shown as ORDER BY 1, 2, 3, ..., n (n = column number of Tables) applied on results. You can see this side effect when you don't have any duplicate row.

I add an example,
UNION, it is merging with distinct --> slower, because it need comparing (In Oracle SQL developer, choose query, press F10 to see cost analysis).
UNION ALL, it is merging without distinct --> faster.
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual
UNION
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual;
and
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual
UNION ALL
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual;

UNION merges the contents of two structurally-compatible tables into a single combined table.
Difference:
The difference between UNION and UNION ALL is that UNION will omit duplicate records whereas UNION ALL will include duplicate records.
Union Result set is sorted in ascending order whereas UNION ALL Result set is not sorted
UNION performs a DISTINCT on its Result set so it will eliminate any duplicate rows. Whereas UNION ALL won't remove duplicates and therefore it is faster than UNION.*
Note: The performance of UNION ALL will typically be better than UNION, since UNION requires the server to do the additional work of removing any duplicates. So, in cases where it is certain that there will not be any duplicates, or where having duplicates is not a problem, use of UNION ALL would be recommended for performance reasons.

Suppose that you have two table Teacher & Student
Both have 4 Column with different Name like this
Teacher - ID(int), Name(varchar(50)), Address(varchar(50)), PositionID(varchar(50))
Student- ID(int), Name(varchar(50)), Email(varchar(50)), PositionID(int)
You can apply UNION or UNION ALL for those two table which have same number of columns. But they have different name or data type.
When you apply UNION operation on 2 tables, it neglects all duplicate entries(all columns value of row in a table is same of another table). Like this
SELECT * FROM Student
UNION
SELECT * FROM Teacher
the result will be
When you apply UNION ALL operation on 2 tables, it returns all entries with duplicate(if there is any difference between any column value of a row in 2 tables). Like this
SELECT * FROM Student
UNION ALL
SELECT * FROM Teacher
Output
Performance:
Obviously UNION ALL performance is better that UNION as they do additional task to remove the duplicate values. You can check that from Execution Estimated Time by press ctrl+L at MSSQL

UNION removes duplicate records in other hand UNION ALL does not. But one need to check the bulk of data that is going to be processed and the column and data type must be same.
since union internally uses "distinct" behavior to select the rows hence it is more costly in terms of time and performance.
like
select project_id from t_project
union
select project_id from t_project_contact
this gives me 2020 records
on other hand
select project_id from t_project
union all
select project_id from t_project_contact
gives me more than 17402 rows
on precedence perspective both has same precedence.

If there is no ORDER BY, a UNION ALL may bring rows back as it goes, whereas a UNION would make you wait until the very end of the query before giving you the whole result set at once. This can make a difference in a time-out situation - a UNION ALL keeps the connection alive, as it were.
So if you have a time-out issue, and there's no sorting, and duplicates aren't an issue, UNION ALL may be rather helpful.

One more thing i would like to add-
Union:- Result set is sorted in ascending order.
Union All:- Result set is not sorted. two Query output just gets appended.

Important! Difference between Oracle and Mysql: Let's say that t1 t2 don't have duplicate rows between them but they have duplicate rows individual. Example: t1 has sales from 2017 and t2 from 2018
SELECT T1.YEAR, T1.PRODUCT FROM T1
UNION ALL
SELECT T2.YEAR, T2.PRODUCT FROM T2
In ORACLE UNION ALL fetches all rows from both tables. The same will occur in MySQL.
However:
SELECT T1.YEAR, T1.PRODUCT FROM T1
UNION
SELECT T2.YEAR, T2.PRODUCT FROM T2
In ORACLE, UNION fetches all rows from both tables because there are no duplicate values between t1 and t2. On the other hand in MySQL the resultset will have fewer rows because there will be duplicate rows within table t1 and also within table t2!

UNION ALL also works on more data types as well. For example when trying to union spatial data types. For example:
select a.SHAPE from tableA a
union
select b.SHAPE from tableB b
will throw
The data type geometry cannot be used as an operand to the UNION, INTERSECT or EXCEPT operators because it is not comparable.
However union all will not.