Query to show dates that are missed in table

This is using SQL Server 2019

Difficult to explain; I have two tables:

DATES:

INSURANCE_COVER:

I need to get all rows from INSURANCE_COVER where the INSURANCE_COVER_ID does not have a INSURANCE_DATE in DATES and I need the missing DATE to show.

I need the returned results to look like this:

I have only been able to do this with a while loop, but the performance is awful.

>Solution :

We can build the list of desired values with a cross join, and then use an exclusion join to find what is missing from that set:

SELECT d.[Date], IDs.INSURANCE_COVER_ID
FROM [dates] d
CROSS JOIN (SELECT DISTINCT INSURANCE_COVER_ID FROM INSURANCE_COVER) IDs
LEFT JOIN INSURANCE_COVER C ON C.INSURANCE_COVER_ID = IDs.INSURANCE_COVER_ID
    AND C.Insurance_Date = d.[Date]
WHERE C.INSURANCE_COVER_ID IS NULL

See it here:

https://dbfiddle.uk/?rdbms=sqlserver_2019&fiddle=d26d09494a0d6b06623a35610170c8eb

We could probably make this even faster with a NOT EXISTS() instead of an exclusion join, but my personal history is I find it much easier and faster to write the exclusion join, and the performance difference doesn’t always justify the additional initial time investment.

As a side note, the date formats in that data are WRONG. Different languages and cultures expect dates formatted in different ways. Some like MM/dd/yyyy. Some like dd-MM-yyyy. Others might prefer yyyy-MM-dd.

The SQL language is no different: it has it’s own expectations around how dates should look, and when writing for SQL you should use that format. When writing a date value with no time, it looks like this: yyyyMMdd (note the complete lack of separators in addition to the ordering). When including a time value, it looks like this: yyyy-MM-ddTHH:mm:ss[.fff].