I have a table like this
Using the COALESCE() function...
SELECT COALESCE(column1, column2, column3) combinedColumn from t;
I get this...
However, I want this....
I found a work around using UNION ALL but this isn't very elegant. Is there a function that works like COALESCE() except includes all values? Thanks
You can't use a coalesce here as there might be more than one value to return
UNION is the best solution (not UNION ALL because blanks)
select column1 from mytable
UNION
select column2 from mytable
UNION
select column3 from mytable
That said, if you want to maintain duplicates (if any), it's UNION ALL, or joining all the columns into a single string then splitting them out again (avoiding the UNION ALL at any cost)
Related
I run a SQL query like below in MySQL:
select *
from (
select 2 as o,1 as t from dual
union
select 1 as o,2 as t from dual
) x
order by if((select 1),o,t);
It works well, but when I use column relative position in if statement, it doesn't work.
How can I use column relative position in if in ORDER BY statement?
select *
from (
select 2 as o,1 as t from dual
union
select 1 as o,2 as t from dual
) x
order by if((select 0),1,2);
I'm not sure what your real confusion is. When an integer appears in an order by, then this is treated as a column number. Any other use of an integer is interpreted as an expression.
The use of column numbers has been removed from the SQL standard. Hence, its use in any particular database is not guaranteed in future releases. It is really better to use the column names.
I think you want to sort your query based on a criteria over two columns, if I'm correct, you can use something like this:
...
order by
case when (your criteria)
then column1
else column2
end;
Note: use union all instead union when you don't want to remove duplicate values as performance issue ;).
Just wanted to ask if this is possible or a way to determine the strings that is not on my table. for example
select name from table_person where name IN('name1','name2','name3')
scenario is name1 and name2 is available on my table but what I want to display is name3, since I want to know what are the things I haven't added.
Just playing around with the worst approach (may be).
Not Recommended
SELECT
suppliedArgTable.name
FROM
(
SELECT 'name1' AS name
UNION
SELECT 'name2'
UNION
SELECT 'name3'
) AS suppliedArgTable
LEFT JOIN
table_person TP ON TP.name = suppliedArgTable.name
WHERE TP.name IS NULL;
http://sqlfiddle.com/#!9/edcbe/2/0
NOT IN combined with reversing your query is a solution.
With the 'list' ('name1','name2','name3') in a (temporary) table e.g. temp_list
and with the data in table_person
the query would be:
select name from temp_list
where name not in (
select distinct(name) from table_person
)
distinct removes doubles. (see also MySQL: Select only unique values from a column)
SELECT name_field
FROM (VALUES('name1'),
('name2'),
('name3'),
('name4'),
('name5')) V(name_field)
EXCEPT
SELECT name_field
FROM name_table
You can use a temporary table to hold a list of all names and find non-matching names with EXCEPT.
My table looks like:
id column1 column2 column3 // feilds names
1 1 0 1 // row
Here I need the columns names of table which contain 1 with respect to "id".
Output I required:
column1 column3
(which has data 1 with respect to id=1).
You haven't mentioned what DBMS you are using; that would be useful. With PostgreSQL you achieve that by using the UNION construct, which is used to combine the result set of multiple SELECT statements:
SELECT id,column1 FROM "Table1" where id=column1
UNION
SELECT id,column2 FROM "Table1" where id=column2
UNION
SELECT id,column3 FROM "Table1" where id=column3;
However, this removes duplicates from the final result set. To also retrieve duplicates use the UNION ALL constrct:
SELECT id,column1 FROM "Table1" where id=column1
UNION ALL
SELECT id,column2 FROM "Table1" where id=column2
UNION ALL
SELECT id,column3 FROM "Table1" where id=column3;
One disadvantage with this approach is that you need as many select statements as there are columns. A second disadvantage is that your final result set will have the column labels of the first select statement (i.e. id, column1 in this example).
I have a simple query that selects one field and only one row, thus one value.
Is there any way to make it return NULL if the query results in an empty set? Instead of returning zero rows?
I think I need to use something with NOT EXISTS, THEN NULL but not certain about it.
select
(Your entire current Select statement goes here) as Alias
from
dual
dual is a built in table with a single row that can be used for purposes like this. In Oracle this is mandatory. MySQL supports it, but you can also just select a single value without specifying a table, like so:
select
(Your entire current Select statement goes here) as Alias
In either case you're selecting a single value. This means that:
If your select returns one value, that value is returned.
If your select statement returns one column, but no rows, NULL will be returned.
If your select statement returns multiple columns and/or multiple rows, this won't work and the query fails.
An easy way to do this is with aggregation:
select max(col)
from t
where <your condition here>
This always returns one row. If there is no match, it returns NULL.
Late reply but I think this is the easiest method:
SELECT
IFNULL((SELECT your query), NULL)
Use a UNION with a NOT EXISTS(original where clause)
select col1
from mytable
where <some condition>
union
select null
where not exists (
select * from mytable
where <some condition>)
You can use COALESCE for example:
SELECT COALESCE(Field1,NULL) AS Field1 FROM Table1
Edit 1:
sorry i mistake with return field as null not result set,for result set return as null use Union and Exist Function like this:
SELECT NULL AS Field1 FROM Table1 WHERE not EXISTS(SELECT Field1 FROM Table1 WHERE Field2>0)
UNION
SELECT Field1 FROM Table1 WHERE Field2>0
Is it possible to join Two queries of mysql in a query ??
Like:
select * from a + select * from b
So that I can use them in a single php loop.
If they have the same number of columns and the datatypes are the same in each column, then you can use a UNION or UNION ALL:
select *
from a
UNION ALL
select *
from b
If you provide more details about the tables, data, etc, then there might be another way of returning this data.
A UNION will return only the DISTINCT values, while a UNION ALL selects all values.
If this is the route that you need to take, and you still need to identify which table the data came from, then you can always create a column to identify which table the data is from , similar to this:
select *, 'a' TableName
from a
UNION ALL
select *, 'b' TableName
from b
This allows you to distinguish what table the data came from.
I think it is easier creating sql "variables" like:
select varA, varb from TableA, tableB;
and you can just play with values in PHP accessing properties.
That way you can take conditions in the query like:
select varA, varb from TableA, tableB where varA.id = varB.foreingId bla bla...
;)