CONVERT Date INT to DATE in SQL - sql

Question

How can I convert a date integer to a date type? (20200531 into 5/31/2020)
My current table has a datadate formatted as YYYYMMDD (20200531, 20200430, etc.)
The Datatype for the datadate is an int according the Toad Data Point software I'm using. I believe it's using ORACLE sql database.
As a result, when querying this data, I have to type in the where clause as below..
where datadate = '20200531'
My goal is to convert this integer datadate into a date format (5/31/2020) so I can apply the datadate to the where clause.
like..
WHERE datadate = dateadd(DD, -1, CAST(getdate() as date))

Answer 1

(Read below for my answer for if it's an int column)
Assuming it's a textual string:
Assuming that datadate is a string (character, text, etc) column and not a date/datetime/datetime2/datetimeoffset column, then use the CONVERT function with style: 23. The 23 value corresponds to ISO 8601 because the values are in yyyy-MM-dd-order, even though they're missing dashes.
This page has a reference of style numbers: https://learn.microsoft.com/en-us/sql/t-sql/functions/cast-and-convert-transact-sql?view=sql-server-ver15
SELECT
*
FROM
(
SELECT
myTable.*
CONVERT( date, datadate, 23 ) AS valueAsDate
FROM
myTable
) AS q
WHERE
q.valueAsDate = DATEADD( dd, -1, GETDATE() )
Assuming it's an actual int column:
The quick-and-dirty way is to convert the int to varchar and then use the same code as above as if it were a textual field - but don't do this because it's slow:
SELECT
*
FROM
(
SELECT
myTable.*,
CONVERT( char(8), datadate ) AS valueAsChar,
CONVERT( date, CONVERT( char(8), datadate ), 23 ) AS valueAsDate
FROM
myTable
) AS q
WHERE
q.valueAsDate = DATEADD( dd, -1, GETDATE() )
Assuming it's an actual int column (better answer):
We'll need to use DATEFROMPARTS and extract each component using Base-10 arithmetic (fun)!
If we have an integer representing a formatted date (the horror) such as 20200531 then:
We can get the day by performing MOD 31 (e.g. 19950707 MOD 31 == 7)
We can get the month by first dividing by 100 to remove the day part, and then MOD 12: (e.g. 20200531 / 100 == 202005, 202005 MOD 12 == 5)
We can get the year by dividing by 10,000, (e.g. 20200531 / 10000 == 2020).
Btw:
SQL Server uses % for the Modulo operator instead of MOD.
Integer division causes truncation rather than producing decimal or floating-point values (e.g. 5 / 2 == 2 and not 2.5).
Like so:
SELECT
q2.*
FROM
(
SELECT
q.*,
DATEFROMPARTS( q.[Year], q.MonthOfYear, q.DayOfMonth ) AS valueAsDate
FROM
(
SELECT
myTable.*,
( datadate % 31 ) AS DayOfMonth,
( ( datadate / 100 ) % 12 ) AS MonthOfYear,
( datadate / 10000 ) AS [Year]
FROM
myTable
) AS q
) AS q2
WHERE
q2.valueAsDate = DATEADD( dd, -1, GETDATE() )
Obviously, having two nested subqueries is a pain to work with (SQL has terrible ergonomics, I don't understand how or why SQL doesn't allow expressions in a SELECT clause to be used by other expressions in the same query - it's really bad ergonomics...) - but we can convert this to a scalar UDF (and SQL Server will inline scalar UDFs so there's no performance impact).
This function has a TRY/CATCH block in it because of the possibility that you process an invalid value like 20209900 (which isn't a real date as there isn't a 99th month with a 0th day in 2020). In this event the function returns NULL.
CREATE FUNCTION dbo.convertHorribleIntegerDate( #value int ) RETURNS date AS
BEGIN
DECLARE #dayOfMonth int = #value % 31;
DECLARE #monthOfYear int = ( #value / 100 ) % 100;
DECLARE #year int = #value / 10000;
BEGIN TRY
RETURN DATEFROMPARTS( #dayOfMonth, #monthOfYear, #year );
END TRY;
BEGIN CATCH
RETURN NULL;
END CATCH;
END
Which we can use in a query like so:
SELECT
myTable.*,
dbo.convertHorribleIntegerDate( datadate ) AS valueAsDate
FROM
myTable
As SELECT cannot share expression results with other expressions in the same query, you'll still need to use an outer query to work with valueAsDate (or repeat the dbo.convertHorribleIntegerDate function call):
SELECT
*
FROM
(
SELECT
myTable.*,
dbo.convertHorribleIntegerDate( datadate ) AS valueAsDate
FROM
myTable
) AS q
WHERE
q.valueAsDate = DATEADD( dd, -1, GETDATE() )

Answer 2

This answers assumes that you are running Oracle, as suggested in your question.
How can I convert a date integer to a date type? (20200531 into 5/31/2020)
In Oracle, you use to_date() to convert a string to a number. If you are giving it a number, it implicitly converts it to a string before converting it. So in both cases, you would do:
to_date(datadate, 'yyyymmdd')
My goal is to convert this integer datadate into a date format (5/31/2020) so I can apply the datadate to the where clause.
Generally, you want to avoid applying a function on a column in a where predicate: it is not efficient, because the database needs to apply the function on the entire column before it is able to filter. If you want to filter on dateadd as of yesterday, then I would recommend computing yesterday's date and putting it in the same format as the column that is filtered, so you can do a direct match against the existing column values.
If your column is a string:
where datadatea = to_char(sysdate - 1, 'yyyymmdd')
If it's a number:
where datadatea = to_number(to_char(sysdate - 1, 'yyyymmdd'))