如何使用转换函数sql（字符串到日期）

Question

I have imported a calendar table from datameer over to my sql server database. I am fairly new to SQL so I do not have the best understanding of the convert function. I have a calendar table with 5 columns (cal_month, cal_year, Toronto, Montreal, National_Avg). The data types are (nvarchar, nvarchar, numeric, numeric, float).

Example of a row in the table would be:

  Month  | Year | Toronto | Montreal | National Average
---------+------+---------+----------+------------------
 January | 2018 | 20      | 21       | 20.5

Below is my attempt at the convert function, my query is supposed to identify current month and -1 to grab the national average from the previous month so in this instance it would be for February 2018.

select convert(date, cal_month), convert(date,cal_year), National_Avg 
from dbo.CALENDAR
where month(cal_month) = month(dateadd(month,-1,current_timestamp))
             and year(cal_year) = year(dateadd(month,-1,current_timestamp))

The error I am receiving is:

Conversion failed when converting date and/or time from character string.

Answer 1

You don't need to manipulate the data in the table. You just need to manipulate your search parameter current_timestamp to fit the data's data-types.

SELECT cal_month, cal_year, National_Avg 
FROM dbo.CALENDAR
WHERE cal_month = DATENAME(mm,   DATEADD(month,-1,current_timestamp))
  AND cal_year  = DATENAME(yyyy, DATEADD(month,-1,current_timestamp))

Answer 2

The problem is probably that you're trying to convert a month/year integer to a date.

Try:

SELECT DATEADD(month, cal_month -1, DATEADD(year, cal_year-1900, 0)) 
from dbo.CALENDAR

Or if you need to get the month/year from today, try

DECLARE @oneMonthAgo datetime 
SET @oneMonthAgo = DATEADD(MONTH, -1, CURRENT_TIMESTAMP) 

DECLARE @cal_month int 
SET @cal_month = MONTH(@oneMonthAgo)

DECLARE @cal_year int 
SET @cal_year = YEAR(@oneMonthAgo)

SELECT National_Avg 
FROM dbo.CALENDAR
WHERE cal_year = @cal_year  AND cal_month  = @cal_month 

Ah, just noticed you use the monthname - that is stupid, but anyway:

DECLARE @cal_month nvarchar(200)
SET @cal_month = DATENAME(month, @oneMonthAgo)

Everything else stays the same.

Or you could add a computed column to the calendar table:

ALTER TABLE dbo.Calendar ADD MonthNum AS  

    CASE Month 
        WHEN 'January' THEN 1 
        WHEN 'February' THEN 2
        WHEN 'March' THEN 3 
        WHEN 'April' THEN 4 
        WHEN 'May' THEN 5
        WHEN 'June' THEN 6 
        WHEN 'July' THEN 7 
        WHEN 'August' THEN 8
        WHEN 'September' THEN 9
        WHEN 'October' THEN 10 
        WHEN 'November' THEN 11 
        WHEN 'December' THEN 12 
    END PERSISTED

That way, you do the lookup faster, as integer.
Which will come in handy as the table fills and gets larger.