T-Sql 101

I mentioned in earlier posts that there’s no standard way to write dates, so we end up having to write them as strings. Now that was a real problem in earlier versions where people would get that wrong.

SQL Server 2012 added an option to make that easier. DATEFROMPARTS allows you to specify a year, month, and a day to create a date, and always in that order.

Look at the following query:

2020-03-23

It’s surprising how often you need to calculate the date for the end of a month i.e. you supply a date, and you want the date for the end of the month that contains that date. The EOMONTH function does that.

Look at the query here:

And the result here:

It might seem really easy to calculate but there are tricky aspects. For example, for a date in February, is it a leap year? You can see from the results that 2020 is a leap year.

2020-03-16

In an earlier post, I showed how you could extract the year, month, day, etc. from dates. But these came back as numbers. So instead of “March”, I got 3 returned.

Sometimes though, I want the name of the period, not the number.

Look at this query:

And you can see what it returns here:

This looks easy, but what if I’m not speaking English? It’s important to understand that functions like DATENAME are affected by the regional/language settings that you are currently using in your session.

2020-03-09

In an earlier post, I showed how you can use the YEAR, MONTH, or DAY functions to extract those parts of a date. But what if you want to extract any one of those other intervals?

Well, that’s what the DATEPART function does. It allows us to specify the interval that we want to extract. Look at the following query examples:

And here are the results of that query:

2020-03-02

When you’re working with T-SQL, it’s common to need to work out how far apart two dates and or times are. Look at the query below:

In this case I’m asking how many days is it from 28th of February 2019 to 31st of July 2019? And if look at the answer:

We can see that it’s 153 days.

Now I used days here but now we could have used any of those other intervals that I discussed when we looked at DATEADD. So I could work at how many minutes that was or how many seconds that was between the two times. I could even say how many months?

2020-02-24

When you’re working with T-SQL, you’ll often need to add time periods onto a date, perhaps onto today’s date. You might be adding (or subtracting) days, or hours, or minutes, or months, and more.

The DATEADD function is the one that we can use add or subtract intervals to the date and time. Now in the example shown here, I’ve started with 20190228, or 28th February 2019. I want to add on 12 days:

2020-02-17

Sometimes we have a date and we need to extract components of the date. For example, we might have a date and wonder what’s the year or we have a date and wonder what the month is or the day.

While there are several ways to extract the year, the month, and/or the day from a date in SQL Server, there are functions designed to do precisely that.

The YEAR function takes a date and extracts the year. The MONTH function takes a date and extracts a month (as a month number from 1 to 12). And the DAY function takes a date and returns the day (again as a day number like 28, not the name of the day like Tuesday).

2020-02-10

Finding out the current time is a really common requirement. But you also have to always consider whose concept of time you’re considering. Is the the time at the server, or the time where you are? (You might not be in the same place, or more importantly, the same timezone).

The function SYSDATETIME() returns the current system date and time right now at the server. The data type that’s returned is a datetime2.  It’s the higher precision datetime data type that was introduced in SQL Server 2008.

2020-02-03

Now, one of the challenges when you go to write dates is that there’s no standard separate format in T-SQL for how to write a date. Instead, we need to write it as a string. So it’s very important to come up with a format that will work properly all the time the example.

In the example I have here, the order date is 20190128. If you use that format (8 digits for a date), it’ll always work OK. SQL Server will take the first four as the year, then 2 for the month, 2 for the day and it does that, no matter what your settings are for your session or for your machine.

2020-01-27

SQL Server has a rich set of data types. Some of them were older data types and a number of new ones were introduced in SQL Server 2008.

One of the important ones was the date data type. It’s just a date. It has no time associated with it and it’s from the year 1 to the year 9999 based upon the Gregorian calendar. Well, they say the Gregorian calendar, even though that’s funny because they was no Gregorian calendar in the year 1. Regardless, for the range of dates that we would work with, this will always work fine.

2020-01-20