Implicit outer apply in SQL - sql

Question

thank you in advance for any assistance you can provide.
I am having to work in Oracle SQL and one issue I'm coming across is how to use applies in SQL. Specifically, hoping someone can help me write the code below to work in SQL. It's not a great example but I'm trying to figure out if there is a good way to:
do implicit applies, referencing columns from the "from" clause table, so they can be used in future applies/joins in the same query without having to do multiple temp tables, CTEs, or inner queries.
same question using top 1 in outer applies
Example:
--Drop Table #Users Drop Table #UsersLoginAttempts
Create table #Users(Username varchar(10), Password1 int)
Create Table #UsersLoginAttempts (Username varchar(10), Password1 int, AttemptDate date)
insert into #Users Values(' MAllen',123), ('SEllis ',124), ('MChen ',126)
insert into #UsersLoginAttempts Values
('MAllen ',123, '20221001'), (' SEllis ',124, '20221001')
, (' MChen ',126, '20221001'), ('MAllen ',126, '20221008')
, (' SEllis ',123, '20221008'), (' MChen',128, '20221008')
Select t.*, u.*
from #Users t
outer apply ( select ltrim(rtrim(t.username)) as UserNameTrim) unt
Outer apply ( select top 1 * from #UsersLoginAttempts ula
where ltrim(rtrim(ula.UserName)) = unt.UserNameTrim
order by ula.AttemptDate desc) u

Answer 1

You do not need the first OUTER APPLY (and would need to add FROM DUAL if you did include it as, in Oracle, all queries must include a table name) and TOP is not supported syntax, you can use FETCH FIRST ROW ONLY instead:
Select t.*,
u.*
from Users u
Outer apply (
select *
from UsersLoginAttempts ula
where TRIM(ula.UserName) = TRIM(u.UserName)
order by ula.AttemptDate desc
FETCH FIRST ROW ONLY
) t
Which, for your sample data:
*Note:
Do not use # as the first character of an identifier (otherwise you will need to use quoted identifiers throughout your code).
Your INSERT syntax is not valid for Oracle; use INSERT ALL ... OR INSERT INTO ... SELECT if you want to insert multiple rows.
Use VARCHAR2 and not VARCHAR.
'20221001' is not a date; it is a string literal. If you want to insert a date then either use TO_DATE, with a format model, to explicitly convert it or use a date literal DATE '2022-10-01'.
Create table Users (
Username varchar2(10),
Password1 int
)
Create Table UsersLoginAttempts (
Username varchar2(10),
Password1 int,
AttemptDate date
)
insert into Users (username, password1)
SELECT ' MAllen', 123 FROM DUAL UNION ALL
SELECT 'SEllis ', 124 FROM DUAL UNION ALL
SELECT 'MChen ', 126 FROM DUAL;
insert into UsersLoginAttempts (username, password1, attemptdate)
SELECT 'MAllen ', 123, DATE '2022-10-01' FROM DUAL UNION ALL
SELECT ' SEllis ', 124, DATE '2022-10-01' FROM DUAL UNION ALL
SELECT ' MChen ', 126, DATE '2022-10-01' FROM DUAL UNION ALL
SELECT 'MAllen ', 126, DATE '2022-10-08' FROM DUAL UNION ALL
SELECT ' SEllis ', 123, DATE '2022-10-08' FROM DUAL UNION ALL
SELECT ' MChen', 128, DATE '2022-10-08' FROM DUAL;
Outputs:
USERNAME
PASSWORD1
ATTEMPTDATE
USERNAME
PASSWORD1
MAllen
126
08-OCT-22
 MAllen
123
 SEllis
123
08-OCT-22
SEllis
124
 MChen
128
08-OCT-22
MChen
126
Note: this is not PL/SQL, which is Oracle Procedural Language (similar to T-SQL for SQL Server), it is just Oracle's dialect of SQL.
fiddle