SQL Server将日期时间与Char比较

Question

I am going over a legacy system where a "date" column report_date in a table t_results is set up as char(25).

Name of the column is report_date and it type is report_date (char(25),null)

A sproc utilizes the table as

ALTER PROCEDURE [dbo].[s_report]
(
    @report_date_from DATETIME,
    @report_date_to DATETIME,
)
AS

SET NOCOUNT ON

SELECT * 
FROM 
    t_results
WHERE
    report_date >= @report_date_from AND report_date <= @report_date_to
ORDER BY report_id

When I run this as EXEC s_report '9/4/2018','10/2/2018' it runs good and I get expected result.

When I run the query below (which is same as sproc), I get no results.

SELECT * 
FROM 
    t_results
WHERE
    report_date >= '09/04/2018' AND report_date <= '10/2/2018'

I am not sure how the sproc is working. Any idea or pointers as to how I compare char with datetime in my where clause?

Answer 1

Your stored procedure is making a conversion to DateTime. When one of the types is different, SQL Server is doing an implicit conversion when it is making the check thus the SP code is implicitly like:

where cast(report_date as datetime) >= '09/04/2018' and cast(report_date as datetime) <= '10/2/2018'

Note: Passing a date string like that is not safe and server settings dependent. Pass like '20180904' and '20181002'.

Answer 2

I think your proc essentially does this:

SELECT * 
FROM t_results
WHERE report_date >= CAST ('09/04/2018' AS DATETIME) 
  AND report_date <= CAST ('10/2/2018' AS DATETIME)