We are expecting null values in a particular column. We would like to capture them in the output also. There are two possible values other than null. They are WE and EA. So, out of these two syntax given below, which one performs better?
…( "Src_Dtl"."REGN" not in ('WE','EA') or
"Src_Dtl"."REGN" is null)…
or
...(coalesce(CVRG_REGN, ‘WE’))...
Thanks in advance.
Null values always returns false for comparisons so not in ('a','b') a null comparison would return false so it would not be included. We are forced to add a 2nd clause or something is null. This is two operations on the data.
The coalesce function says give me the first non null value in the list. It is one operation (check if the value is null) and has two possible outcomes. Because this is one operation it is faster than doing two operations.
Related
I'm using SQL Server, and I have a table with two columns, both varchar, column A and column B. I need to produce a list in alphabetical order, however only one of these columns will ever have a value in it (ie, if column A has a value, then column B will be NULL and vice versa).
How can I write an ORDER BY clause in my T-SQL query to produce a list that checks both columns to see which one has the value present, then order the rows alphabetically?
Use COALESCE which takes the first non-null argument
order by coalesce(columnA, columnB) asc
There are some standard options to do this. What you choose is mostly personal "taste". The most "explicit" way is using CASE WHEN:
ORDER BY CASE WHEN columnA IS NULL THEN columnB ELSE columnA END;
By explicit, I mean you clearly understand it without knowing about specific functions that check this.
The standard function to do this which works on every DB is COALESCE:
ORDER BY COALESCE(columnA,columnB);
This has the advantage it's much shorter, especially when you have more columns that should replace each other when null.
SQL Server DB furthermore provides the function ISNULL that expects exact two arguments:
ORDER BY ISNULL(columnA,columnB);
The advantage of this is the name tells a bit more than "COALESCE", also it is faster than the other two options according to some performance articles and tests. The disadvantage is this function will not work on other DB's.
Overall, as I said, it's mainly kind of personal taste which option you should take.
I am currently working on a data flow and have been given a specific requirement that i am trying to complete.
In my table I have a column which is partially NULL due to a couple of reasons.
What I'm trying to do is write a case expression within my select statement that has two conditions:
When NULL use a different value from another column (which is pulled from another table using a join)
If the column is still NULL (in both cases) then use a different value from another column in the table which will ensure the column is populated.
So basically, if it's NULL do this, if its still NULL, then do this which will mean my column is populated as i intend.
I've been playing around but have been unable to produce the required result. Is this something that can be achieved using a CASE expression ?
Any help/advice would be appreciated.
Thanks.
You are describing the coalesce() function:
coalesce(col1, col2, col3)
You can use this in either a select or update.
My professor is teaching sql 2, and used an statement like the code below into a query:
HAVING SUM(column) > subselect
Where subselect is something like SELECT AVG(column) FROM ...
This subselect returns only one value, but I could not understand how is it possible to compare a function (the sum) with a subselect. The subselect should return a table, right? Then how is it possible to compare a table with a value? That did not make sense to me.
Thanks in advance.
SQL has a concept of scalar subqueries. These are subqueries that return exactly one column and at most one row. Scalar subqueries can be used in almost all cases where a single value ("scalar") can be used.
If the scalar subquery returns no rows, then the value is treated as NULL.
(I should add that some databases support tuples. A tuple is a set of scalar values that is treated as a single value. In such databases "scalar" subqueries can return more than one value, but these are converted to a tuple. This is not relevant to the question being asked; tuples are just another example of a "single" value.)
In principle you are right, if you look at it in a relational way.
But SQL is an industry standard and allows the abbreviation of comparing a scalar value with a result table with only a single row and column.
Depending on the exact implementation, it even allows to compare a scalar with a list of values (a column with more than one value), although you actually should write ... value > ALL (subselect), it is often accepted without the ALL keyword.
This is both valid syntax in the where and in the having clause
I have a where clause that compares two columns to the following string. Does this concatenation run for every row? Should this string (and concatenation) be left in twice or should I create a variable to hold the result and use that in the WHERE clause?
CONCAT('%', #myVar, 'dr')
I checked the Execution Plan for your expression in a test table present in my database. The table has two nvarchar(50) columns namely firstname and fullname .
I can clearly see that even for three AND conditions in where clause that I put deliberately, the SQL Server engine is showing 0% cost for both the compute scalar steps. It is clearly evident of the fact that irrespective of whether you create a separate variable for the concatenation expression or leave it in-line in your where clause it is not going to make any difference.
Create a variable to hold the result and use that in the WHERE clause
I am using a PostgreSQL database, and in a table representing some measurements I've two columns: measurement, and interpolated. In the first I've the observation (measurement), and in the second the interpolated value depending on nearby values. Every record with an original value has also an interpolated value. However, there are a lot of records without "original" observations (NULL), hence the values are interpolated and stored in the second column. So basically there are just two cases in the database:
Value Value
NULL Value
Of course, it is preferable to use the value from the first column if available, hence I need to build a query to select the data from the first column, and if not available (NULL), then the database returns the value from the second column for the record in question. I have no idea how to build the SQL query.
Please help. Thanks.
You can use Coalesce:
The COALESCE function returns the first of its arguments that is not null. Null is returned only if all arguments are null.
Select Coalesce( first_value, second_value )
From your_table
This would return first_value if it is not NULL, second_value otherwise.
Peter nailed it. But for the sake of completeness (the title of your question is more general than your particular issue), here goes the docs for the several conditional expressions available in Postgresql (and in several other databases) : CASE, COALESCE, NULLIF, GREATEST, LEAST. The first one is the most general.