The SQL SELECT instruction is the most important single thing to master
It is the instruction that actually gives the answers to any questions
It can have up to six sections, as follows
SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY
But some of these are used a lot less often than others
SELECT lists the things we want to see
FROM specifies which places to get these from
WHERE specifies which things we do or don't want to see
GROUP BY allows for some grouping or totalling up
HAVING allows us to specify some more things we do or don't want if we used GROUP BY
ORDER BY specifies which order to list the results in
SELECT can be used on it's own without any of the other sections. There are relatively few sensible uses for this. One might be to see what the computer thinks the date and time is
SELECT Getdate()
gives results:
Getdate() is a 'function'. More on these later
To be of much use, we have to SELECT from one or more tables.
The simplest use of SELECT is with the * (star) symbol which means 'Select Everything'
SELECT *
FROM [TESTDB].[dbo].[tblExampleTable]
gives results:
This is little caricature of real data which we can use for illustrating SQL. It is in a table called tblExampleTable. The [TESTDB].[dbo].bit gives more information on exactly where this table is
So the SELECT * lists all five records in full. The order it comes back in is the not controlled.
We may want to have the list sorted alphabetically. The ORDER BY section can be used for this
SELECT *
FROM [TESTDB].[dbo].[tblExampleTable]
ORDER BY [Description]
gives
Or we might want it sorted by the (in this case meaningless) number data, with the highest first. So we would add DESC (short for Descending) to the ORDER BY bit as follows
SELECT *
FROM [TESTDB].[dbo].[tblExampleTable]
ORDER BY [SomeNumber] DESC
which gives:
Often we will not want every field in the table shown in the results. So rather than SELECT * we can SELECT specific fieldnames
SELECT [Description], [SomeOtherThing]
FROM [TESTDB].[dbo].[tblExampleTable]
More often than not we will be interested in looking at a specific part of the data rather than seeing everything each time. We can use the WHERE section to limit which records are shown. For example
SELECT [Description], [SomeOtherThing]
FROM [TESTDB].[dbo].[tblExampleTable]
WHERE [SomeOtherThing]='Green'
gives
or
SELECT [Description], [SomeOtherThing]
FROM [TESTDB].[dbo].[tblExampleTable]
WHERE [SomeOtherThing]<>'Green'
gives
The <> means 'is not equal to' so is basically 'everything but'
It is not necessary for the data items used for selecting or listing to be displayed in the final results
That is the end of Part 2
In Part 3 we will look at counting
