Multiple SQL SELECT - sql

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.

Related

Oracle union multiple tables with same sub-query

I have an inefficiently written Oracle SQL query with multiple unions of sub-queries (about 7 or 8 tables) that differ only by the table queried in sub-query, that I am certain can be written more succinctly. Observe the code at bottom, in that the only difference between the union-ed sub-queries is the table names (in this case table_a / table_b).
SELECT
/*+ parallel(10) */
Col_Alpha,
Col_Beta,
Col_Gamma,
Col_Delta,
col_epsilon
FROM
table_a
WHERE
Col_Theta = 'CAT'
And Col_Kappa In ('CAR','TRUCK','PLANE')
UNION
SELECT
/*+ parallel(10) */
Col_Alpha,
Col_Beta,
Col_Gamma,
Col_Delta,
col_epsilon
FROM
table_b
WHERE
Col_Theta = 'CAT'
AND col_kappa IN ('CAR','TRUCK','PLANE')
I tried giving a list of tables after the from clause but that did not work. I also added variations of:
from
(table_a, table_b)
And that did not work. I have tried finding a way to compress the code but I do not know enough for a successful search.
I cannot use procedures with my level of access.
I expect an output similar to what I'm getting, a union of several tables with the same columns queried and same filters across all of them, but that takes around 1/7 the amount of code.

Unfortunately you can't get away from multiple SELECT statements. The way UNION is designed to work according to Oracle: "You can combine multiple queries using the set operators UNION, UNION ALL, INTERSECT, and MINUS."
In other words, you need to HAVE multiple queries in order to UNION them.
But you can move the conditions to outside the UNION so that you only have to repeat those once:
SELECT * FROM(
Select col1, col2, col3 FROM table_1 UNION
Select col1, col2, col3 FROM table_2 UNION
Select col1, col2, col3 FROM table_3 UNION
Select col1, col2, col3 FROM table_4)
WHERE Col_Theta = 'CAT'
AND Col_Kappa In ('CAR','TRUCK','PLANE');
You can also throw in ORDER BY and other conditions at the end as well. This doesn't remove all of the repetition but at least makes it so that if you change the WHERE conditions you're only changing them in one place.

postgres: Querying across rows in multiple tables?

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';

Combine results of two table in SQL

i have two tables one contain computer_science_book and Mechanical_engineering_book i want the result which contail all books of computer_science_book and Mechanical_engineering_book .
give me suggetions.

You can use union all for that:
select * from computer_science_book
union all
select * from Mechanical_engineering_book

You can use the UNION operator:
select columns from computer_science_books
UNION
select columns from mechanical_eng_books
Note that the structure of the results from computer_science_books needs to match the structure of results from mechanical_eng_books. The reason is that, as asked for, there is only one result table. There is also the UNION ALL operator. The difference is that UNION removes duplicates whereas UNION ALL returns all combined rows.

Is there some way to do the following in SQL?

Let's imagine that we have these 2 tables:
Table 1, with the column:
Field1
1
3
Table 2, with the column:
Field1
2
4
(Well they could also be called in any other way, but I want to represent that the type of table1.field1 is the same as table2.field1).
Would it be possible to do a SQL query that would return the following?
[1,2,3,4], I mean the numbers ordered by any criteria I would want but that criteria aplying to both tables. As far as I know ORDER BY can just ORDER by the values of a column, not by a general criteria like "from lower to higher number. And even if it could I believe the SELECT instruction can't fuse columns. I mean I think the best I could achieve with that instruction would be to get something like [(1,2),(1,4),(3,2),(3,4)] and later work on it, but this can be painful with lots of results.
And the application needs fields to be on different tables, I cannot merge them.
Any idea about how to deal with this?
Thanks a lot for your help.
Edit:
Oh, it was much easier than what I thought, with that instruction is not something hard to achieve.
Thank you everyone.

This is what the UNION statement is for. It lets you combine two SELECT statements into the same resultset:
SELECT Field1
FROM Table1
UNION ALL
SELECT Field1
FROM Table2
ORDER BY 1

can you do union all
Like below:
Select field 1
from
(Select field 1 from Table 1
Union
select field 1 from table 2)
order by field 1
Use union or Union all based on your need to repeat elements in both the tables or not.

select * from
(
select field1 as field_value from table1
union
select field2 as field_value from table2
)
order by field_value asc

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.