如何将nvarchar转换为整数？

Question

I have a database I'm running queries on where I cannot change the schema. I am in my second year of database management. We have not touched much on writing actual SQL as opposed to just using the GUI to create our queries and manage our DB's.

I have a population attribute that I need to run SUM on but the population is datatype ncharvar. I need to cast it to int.

I don't know how to do that in SQL though! Can someone please show me? I've been fiddling with it for awhile and I'm out of ideas. I'm very unfamiliar with SQL (as simple as it looks) and this would be helpful.

SELECT dbo_City.CityName, Sum(dbo_City.Population) AS SumOfPopulation
FROM dbo_City
GROUP BY dbo_City.CityName
ORDER BY dbo_City.CityName, Sum(dbo_City.Population);

I need to find what cities have populations between 189,999 and 200,000, which is a very simple query. I'm grouping by city and using the sum of the population. I'm not sure where to insert the 189,999-200,000 figure in the query but I can figure that out later. Right now I'm stuck on casting the ncharvar Population field to an Int so I can run this query!

I found the answer here:

Using SUM on nvarchar field

SELECT SUM(CAST(NVarcharCol as int))

But I'm not sure how to execute this solution. Specifically, I'm not sure where to insert this code in the above provided SQL, and I don't understand why the nvarchar is called nvarchar col .

Answer 1

From MSDN:

Syntax for CAST: CAST ( expression AS data_type [ ( length ) ] )

Your solution should look something like this:

SELECT c.CityName, CAST(c.Population AS INT) AS SumOfPopulation
FROM dbo_City AS c
WHERE ISNUMERIC(c.Population) = 1 AND CAST(c.Population AS INT) BETWEEN 189999 AND 200000
ORDER BY c.CityName, CAST(c.Population AS INT)

You shouldn't need the sum function unless you want to know the total population of the table, which would be more useful if it was a table of countries, cities, and city populations, unless this particular city table is broken down further (such as with individual zip codes?). In that case, the below would be the preference:

SELECT c.CityName, SUM(CAST(c.Population AS INT)) AS SumOfPopulation
FROM dbo_City AS c
WHERE ISNUMERIC(c.Population) = 1
GROUP BY c.CityName
HAVING SUM(CAST(c.Population AS INT)) BETWEEN 189999 AND 200000
ORDER BY c.CityName, SUM(CAST(c.Population AS INT))

I hope this helps point you in the right direction.

-C§

Edit: Integrated the "fail safe" from your linked syntax, which should stop that error coming up. It adds a filter to the column to only process those that are able to be cast to a numeric type without extra processing (such as removing the comma as in vkp's response).

Answer 2

I ran into a similar problem where I had temperatures (temp) stored as nvarchar. I wanted to filter out all temperatures under 50F. Unfortunately, WHERE (temp > '5') would include temperatures that started with a - sign (-5, -6, ...); even worse, I discovered that temperatures over 100F were also getting discarded.

SOLUTION: WHERE (CAST (temp as SMALLINT) > '50')

I know this doesn't "directly" answer your question but for the life of me I couldnt find a specific answer to my problem anywhere on the web. I thought it would be lame to answer my own question, so I wanted to add that my discovery to your answer.