sql - SELECT COUNT(*) ;

Question

I have a database, database1, with two tables (Table 1, Table2) in it.

There are 3 rows in Table1 and 2 rows in Table2. Now if I execute the following SQL query SELECT COUNT(*); on database1, then the output is "1".

Does anyone has the idea, what this "1" signifies?

The definition of the two tables is as below.

CREATE TABLE Table1
(
ID INT PRIMARY KEY,
NAME NVARCHAR(20)
)

CREATE TABLE Table2
(
ID INT PRIMARY KEY,
NAME NVARCHAR(20)
)

Answer 1

Normally all selects are of the form SELECT [columns, scalar computations on columns, grouped computations on columns, or scalar computations] FROM [table or joins of tables, etc]

Because this allows plain scalar computations we can do something like SELECT 1 + 1 FROM SomeTable and it will return a recordset with the value 2 for every row in the table SomeTable.

Now, if we didn't care about any table, but just wanted to do our scalar computed we might want to do something like SELECT 1 + 1. This isn't allowed by the standard, but it is useful and most databases allow it (Oracle doesn't unless it's changed recently, at least it used to not).

Hence such bare SELECTs are treated as if they had a from clause which specified a table with one row and no column (impossible of course, but it does the trick). Hence SELECT 1 + 1 becomes SELECT 1 + 1 FROM ImaginaryTableWithOneRow which returns a single row with a single column with the value 2.

Mostly we don't think about this, we just get used to the fact that bare SELECTs give results and don't even think about the fact that there must be some one-row thing selected to return one row.

In doing SELECT COUNT(*) you did the equivalent of SELECT COUNT(*) FROM ImaginaryTableWithOneRow which of course returns 1.