On 11/5/17, Shane Dev <[email protected]> wrote:
>
> In sqlite3, I executed the following statements -
>
> sqlite> select name from tabs where rowid=1;
> tab1
> sqlite> select * from tab1;
> first rec
>
> sqlite> select * from (select name from tabs where rowid=1);
> tab1
>
> I expected the last statement to evaluate the subquery first to be 'tab1'
> and then execute SELECT * FROM tab1 to yield 'first rec'. Why didn't this
> happen?
Because that is not the way SQL works. The statement
SELECT * FROM (SELECT name FROM tabs WHERE rowid=1);
is logically equivalent to:
CREATE TEMP TABLE "some-random-name" AS
SELECT name FROM tabs WHERE rowid=1;
SELECT * FROM "some-random-name";
DROP TABLE "some-random-name";
SQL works on a compile-then-execute model. Each SQL statement is
first analyzed and compiled into bytecode or into machine code or some
other executable format. Then the resulting compiled code is run to
generate a result. The names of tables and columns are fixed at
compile-time and cannot be modified at runtime, since to do so would
require on-the-fly changes to the compiled code.
--
D. Richard Hipp
[email protected]
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