In the last lesson we established a minimal foundation of SQL knowledge Now it's time to cover how you get data from your database using SQL.
In SQL, you don't GRAB data, you don't TAKE data, you certainly don't INHALE data (that could get you in trouble with the Feds). Sometimes you FETCH data, but that's an advanced use. Instead, you SELECT data.
Let's continue our patient chart example from the first lesson. Let's say we have a "Patients" table. For each patient, we're storing their first and last names, their heart rate (pulse), and the last date they were examined.
We might name our columns like so:
If we wanted to retrieve every chart and every bit of data for each chart, the SELECT statement would look like this:
SELECT * FROM Patients
Basically the asterisk (*) is a shortcut that means "all columns".
So, for every record in our table, we'll be grabbing the FirstName,
LastName, Pulse, and LastVisit columns.
While using the asterisk (some call it a "splat") is faster to type,
it is considered best practice to name each of the columns you want to return.
So, let's rewrite our statement:
SELECT FirstName, LastName, Pulse, LastVisit FROM Patients
It's longer, but more explicit and maybe a tad more efficient. A key point to remember is that you must
always tell the server what columns you want it to return - whether explicitly as we've been doing or implicitly
via the asterisk shortcut.
The above SELECT statement might return data like this:
FYI: Lest our international readers get confused by the dates, I'm using US date formatting which is month/day/year
Notice I'm using the same order of columns that I defined in the table originally. It doesn't have to be this way. I could say:
SELECT LastName, LastVisit, Pulse, FirstName FROM Patients
The columns will be returned like so: