Using MSSQL to strtodate date ranges

gistfile1.txt

DECLARE @StartDate DATETIME, @EndDate DATETIME
 
SELECT
    @StartDate = 
    CASE WHEN 'Today' THEN CAST(FLOOR(CAST(GetDate() AS FLOAT)) AS DATETIME)
    CASE WHEN 'This Week' THEN DATEADD(dd, -1 * (DATEPART(dw, GETDATE()) - 1), CAST(FLOOR(CAST(GETDATE() AS FLOAT)) AS DATETIME))
    CASE WHEN 'ThisMonth' THEN DATEADD(dd, -1 * (DATEPART(d, GETDATE()) - 1), CAST(FLOOR(CAST(GETDATE() AS FLOAT)) AS DATETIME))
    ELSE '1/1/1900'
    END
 
SELECT
    @EndDate = 
    CASE WHEN 'Today' THEN DATEADD(d, 1, CAST(FLOOR(CAST(GetDate() AS FLOAT)) AS DATETIME))
    CASE WHEN 'This Week' THEN DATEADD(d, 7, DATEADD(dd, -1 * (DATEPART(dw, GETDATE()) - 1), CAST(FLOOR(CAST(GETDATE() AS FLOAT)) AS DATETIME)))
    CASE WHEN 'ThisMonth' THEN DATEADD(m,1,DATEADD(dd, -1 * (DATEPART(d, GETDATE()) - 1), CAST(FLOOR(CAST(GETDATE() AS FLOAT)) AS DATETIME)))
    ELSE '12/31/9999'
    END
 
SELECT * FROM articles WHERE creation_date >= @StartDate AND creation_date < @EndDate
 

Can you globalize variables in MySQL like this?

Yes, it needs to be enclosed in a BEGIN .... END block as a stored procedure or trigger.