Related
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.
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.
I'm new to MySql and I need to append the results of two independently ordered queries, limit the result of the whole thing combined, and return starting at a certain index.
So something like this
the first query will result in an ordered list like this
a
b
c
the second query will result in another ordered list like this
1
2
3
Then append the second result after the first like this
a
b
c
1
2
3
Then only return a certain number of elements and start at a particular index, so for example I only want total 4 return values starting at index 2, the result should be
c
1
2
3
As you can see, "c" is at index 2 of the appended result, and 4 elements are returned.
Anybody knows how to write this complicated MySQL statement?
For the first part of your problem, you could use a UNION
UNION is used to combine the result from multiple SELECT statements into a single result set.
Read more here
As for the second part of your problem, you could use LIMIT. Read this SO Answer
Now to combine the both, lets say you have two tables - tbl1 and tbl2 as
tbl1 tbl2
a 1
b 2
c 3
You could use a query as follows:
SELECT * FROM (SELECT * FROM tbl1 UNION SELECT * FROM tbl2) as t
LIMIT 2, 9430903;
The second number in the limit clause is just a very large number. So this query will retrieve all rows from the 3rd row to the last row.
Hope this helps.
Lets say I have a table with a column of ages..
Here is the list of ages
1
2
3
1
1
3
I want the SQL to count how many of age 1s, how many of 2s and 3s.
The code:
Select count(age) as age1 where age = ‘1’;
Select count(age) as age2 where age = ‘2’;
Select count(age) as age3 where age = ‘3’;
Should work but would there be a way to just display it all using only 1 line of code?
This is an instance where the GROUP BY clause really shines:
SELECT age, COUNT(age)
FROM table_name
GROUP BY age
Just an additional tip:
You shouldn't use single quotes here in your query:
WHERE age = '1';
This is because age is an INT data type and therefore does not have single quotes. MySQL will implicitly convert age to the correct data type for you - and it's a negligible amount of overhead here. But imagine if you were doing a JOIN of two tables with millions of rows, then the overhead introduced would be something to consider.
Try this ,if the count is limited to three ages ,also using aggregate functions without grouping them will result in a single row,you can use SUM() with the condition which will result in a boolean and you can get the count based on your criteria
Select SUM(age = '1') as age1,
SUM(age = '2') as age2,
SUM(age = '3') as age3
from table
SELECT SUM(CASE WHEN age = 1 THEN 1 ELSE 0 END) AS age1,
SUM(CASE WHEN age = 2 THEN 1 ELSE 0 END) AS age2,
SUM(CASE WHEN age = 3 THEN 1 ELSE 0 END) AS age3
FROM YourTable
If your query should return only one column (age in this case, you can use Count+groupby):
SELECT age, Count(1) as qty
FROM [yourTable]
GROUP BY age
Remember you must include any additional column in your group by condition.
Select age as Age_Group, count(age) as Total_count from table1 group by age;
select age, count(age) from SomeTable group by age
http://sqlfiddle.com/#!2/b40da/2
The group by clause works like this:
When using aggregate functions, like the count function without a group by clause the function will apply to the entire dataset determined by the from and where clauses. A count will for instance count the number of rows in the result set, and sum over a specfic column will sum all the rows in the result set.
What the group by clause allows us to do, is to divide the result set determined by the from and where clause into partitions, so that the aggregate functions no longer applies to the result set as a whole, but rather within each partition of the result set.
When you specify a column to group by, what you are saying is something like "for each distinct value of column x in the result set, create a partition containing any row in the result set with this particular value in column x". Then, instead of yielding one result covering the entire resultset, aggregate functions will yield one result for each distinct value of column x in the result set.
With your example input of:
1
2
3
1
1
3
let's analyze the above query. As always, we should look at the from clause and the where clause first. The from clause tells us that we are selecting from SomeTable and only this, and the lack of a where clause tells us that we are selecting from the full contents of SomeTable.
Next, we'll look at the group by clause. It's present, and it groups by the age column, which is the only column in our example. The presence of the group by clause changes our dataset completely! Instead of selecting from the entire row set of SomeTable, we are now selecting from a set of partitions, one for each distinct value of the age-column in our original result set (which was every row in SomeTable).
At last, we'll look at the select-clause. Now, since we are selecting from partitions and not regular rows, the select-clause has fewer options for what it can contain, actually it only has 2: The column that it is grouped by, or an aggregate function.
Now, in our example we only have one column, but consider that we had another column, like here:
http://sqlfiddle.com/#!2/d5479/2
Now, imagine that in our data set we have two rows, both with age='1', but with different values in the other column. If we were to include this other column in a query that is grouped by the age-column (which we now know will return one row for each partition over the age-column), which value should be presented in the result? It makes no sense to include other column than the one you grouped by. (I'll leave multiple columns in the group by clause out of this, in my experience one usually just wants one..)
But back to our select-clause, knowing our dataset has the distinct values {1, 2, 3} in the age-column, we should expect to get 3 rows in our result set. The first thing to be selected is the age-column, which will yield the values [1, 2, 3]´ in the three rows. Next in theselect-list is an aggregate functioncount(age), which we now know will count the number of rows in each partition. So, for the row in the result whereage='1', it will count the number of rows withage='1', for the row whereage='2'it will count the number of rows whereage='2'`, and so on.
The result would look something like this:
age count(age)
1 3
2 1
3 2
(of course you are free to override the name of the second column in the result, with the as-operator..)
And that concludes today's lesson.
The issue here is suppose if i want to use two queries seperated by UNION, the query is as
$query=(select a.name,a.age,b.country,b.state from a,b where a.aid=b.bid) UNION (select a.name,a.age,c.profession,c.salary from a,c where a.anid=c.cid)
here the result would only show the first query's result , Any way in which i could display the result of 2nd query also down to the result of first query using UNION. Expecting any help on this. Thanks
Are you after
(
select a.name,a.age,b.country,b.state,null as profession,null as salary
from a,b
where a.aid=b.bid
)
UNION
(
select a.name,a.age,null,null,c.profession,c.salary
from a,c
where a.anid=c.cid
)
You will have null in the profession and salary columns from the first query and null in country and state columns in the second query
Try this
$query=(select a.name,a.age,b.country,b.state from a,b where a.aid=b.bid UNION select a.name,a.age,c.profession,c.salary from a,c where a.anid=c.cid)
by the way the fields in both select must be the same datatype
As I understood it a union was to do the same query on two different tables. If you get a result from the first half of the union you will not get the results from the second half.
Basic property of UNION is
Selected columns listed in
corresponding positions of each SELECT
statement should have the same data
type. (For example, the first column
selected by the first statement should
have the same type as the first column
selected by the other statements.)
If the data types of corresponding
SELECT columns do not match, the types
and lengths of the columns in the
UNION result take into account the
values retrieved by all of the SELECT
statements. For example, consider the
following: