Question

How does SQLite internally treats the alias?

Does creating a table name alias internally creates a copy of the same table or does it just refers to the same table without creating a copy?

When I create multiple aliases of the same table in my code, performance of the query is severely hit!

In my case, I have one table, call it MainTable with namely 2 columns, name and value.
I want to select multiple values in one row as different columns. for example

Name: a,b,c,d,e,f
Value: p,q,r,s,t,u
such that a corresponds to p and so on.

I want to select values for names a,b,c and d in one row => p,q,r,s
So I write a query

SELECT t1.name, t2.name, t3.name, t4.name  
FROM MainTable t1, MainTable t2, MainTable t3, MainTable t4
WHERE t1.name = 'a' and t2.name = 'b' and t3.name = 'c' and t4.name = 'd';

This way f writing the query kills the performance when size of the table increases as rightly pointed above by Larry.

Is there any efficient way to retrieve this result. I am bad at SQL queries 🙁

Answer 1

Assuming you have table dbo.Customers with a million rows

SELECT * from dbo.Customers A

does not result in a copy of the table being created.

As Larry pointed out, the query as it stands is doing a cartesian product across your table four times which, as you has observed, kills your performance.

The updated ticket states the desire is to have 4 values from different queries in a single row. That’s fairly simple, assuming this syntax is valid for sqllite

You can see that the following four queries when run in serial produce the desired value but in 4 rows.

SELECT t1.name
FROM MainTable t1
WHERE t1.name='a';

SELECT t2.name
FROM MainTable t2
WHERE t2.name='b';

SELECT t3.name
FROM MainTable t3
WHERE t3.name='c';

SELECT t4.name
FROM MainTable t4
WHERE t4.name='d';

The trick is to simply run them as sub queries like so there are 5 queries: 1 driver query, 4 sub’s doing all the work. This pattern will only work if there is one row returned.

SELECT
(
    SELECT t1.name
    FROM MainTable t1
    WHERE t1.name='a'
) AS t1_name
,
(
    SELECT t2.name
    FROM MainTable t2
    WHERE t2.name='b'
) AS t2_name
,
(
    SELECT t3.name
    FROM MainTable t3
    WHERE t3.name='c'
) AS t3_name
, 
(
    SELECT t4.name
    FROM MainTable t4
    WHERE t4.name='d'
) AS t4_name