Order of JOIN and WHERE [duplicate] - mysql

Question

I am confused with the order of execution of this query, please explain me this.
I am confused with when the join is applied, function is called, a new column is added with the Case and when the serial number is added. Please explain the order of execution of all this.
select Row_number() OVER(ORDER BY (SELECT 1)) AS 'Serial Number',
EP.FirstName,Ep.LastName,[dbo].[GetBookingRoleName](ES.UserId,EP.BookingRole) as RoleName,
(select top 1 convert(varchar(10),eventDate,103)from [3rdi_EventDates] where EventId=13) as EventDate,
(CASE [dbo].[GetBookingRoleName](ES.UserId,EP.BookingRole)
WHEN '90 Day Client' THEN 'DC'
WHEN 'Association Client' THEN 'DC'
WHEN 'Autism Whisperer' THEN 'DC'
WHEN 'CampII' THEN 'AD'
WHEN 'Captain' THEN 'AD'
WHEN 'Chiropractic Assistant' THEN 'AD'
WHEN 'Coaches' THEN 'AD'
END) as Category from [3rdi_EventParticipants] as EP
inner join [3rdi_EventSignup] as ES on EP.SignUpId = ES.SignUpId
where EP.EventId = 13
and userid in (
select distinct userid from userroles
--where roleid not in(6,7,61,64) and roleid not in(1,2))
where roleid not in(19, 20, 21, 22) and roleid not in(1,2))
This is the function which is called from the above query.
CREATE function [dbo].[GetBookingRoleName]
(
#UserId as integer,
#BookingId as integer
)
RETURNS varchar(20)
as
begin
declare #RoleName varchar(20)
if #BookingId = -1
Select Top 1 #RoleName=R.RoleName From UserRoles UR inner join Roles R on UR.RoleId=R.RoleId Where UR.UserId=#UserId and R.RoleId not in(1,2)
else
Select #RoleName= RoleName From Roles where RoleId = #BookingId
return #RoleName
end

Answer 1

Queries are generally processed in the follow order (SQL Server). I have no idea if other RDBMS's do it this way.
FROM [MyTable]
ON [MyCondition]
JOIN [MyJoinedTable]
WHERE [...]
GROUP BY [...]
HAVING [...]
SELECT [...]
ORDER BY [...]

Answer 2

SQL is a declarative language. The result of a query must be what you would get if you evaluated as follows (from Microsoft):
Logical Processing Order of the SELECT statement
The following steps show the logical
processing order, or binding order,
for a SELECT statement. This order
determines when the objects defined in
one step are made available to the
clauses in subsequent steps. For
example, if the query processor can
bind to (access) the tables or views
defined in the FROM clause, these
objects and their columns are made
available to all subsequent steps.
Conversely, because the SELECT clause
is step 8, any column aliases or
derived columns defined in that clause
cannot be referenced by preceding
clauses. However, they can be
referenced by subsequent clauses such
as the ORDER BY clause. Note that the
actual physical execution of the
statement is determined by the query
processor and the order may vary from
this list.
FROM
ON
JOIN
WHERE
GROUP BY
WITH CUBE or WITH ROLLUP
HAVING
SELECT
DISTINCT
ORDER BY
TOP
The optimizer is free to choose any order it feels appropriate to produce the best execution time. Given any SQL query, is basically impossible to anybody to pretend it knows the execution order. If you add detailed information about the schema involved (exact tables and indexes definition) and the estimated cardinalities (size of data and selectivity of keys) then one can take a guess at the probable execution order.
Ultimately, the only correct 'order' is the one described ion the actual execution plan. See Displaying Execution Plans by Using SQL Server Profiler Event Classes and Displaying Graphical Execution Plans (SQL Server Management Studio).
A completely different thing though is how do queries, subqueries and expressions project themselves into 'validity'. For instance if you have an aliased expression in the SELECT projection list, can you use the alias in the WHERE clause? Like this:
SELECT a+b as c
FROM t
WHERE c=...;
Is the use of c alias valid in the where clause? The answer is NO. Queries form a syntax tree, and a lower branch of the tree cannot be reference something defined higher in the tree. This is not necessarily an order of 'execution', is more of a syntax parsing issue. It is equivalent to writing this code in C#:
void Select (int a, int b)
{
if (c = ...) then {...}
int c = a+b;
}
Just as in C# this code won't compile because the variable c is used before is defined, the SELECT above won't compile properly because the alias c is referenced lower in the tree than is actually defined.
Unfortunately, unlike the well known rules of C/C# language parsing, the SQL rules of how the query tree is built are somehow esoteric. There is a brief mention of them in Single SQL Statement Processing but a detailed discussion of how they are created, and what order is valid and what not, I don't know of any source. I'm not saying there aren't good sources, I'm sure some of the good SQL books out there cover this topic.
Note that the syntax tree order does not match the visual order of the SQL text. For example the ORDER BY clause is usually the last in the SQL text, but as a syntax tree it sits above everything else (it sorts the output of the SELECT, so it sits above the SELECTed columns so to speak) and as such is is valid to reference the c alias:
SELECT a+b as c
FROM t
ORDER BY c;

Answer 3

SQL query is not imperative but declarative, so you have no idea which the statement is executed first, but since SQL is evaluated by SQL query engines, most of the SQL engines follows similar process to obtain the results. You may have to understand how the query engine works internally to understand some SQL execution behavior.
Julia Evens has a great post explaining this, it is worth to check it out:
https://jvns.ca/blog/2019/10/03/sql-queries-don-t-start-with-select/

Answer 4

SQL is a declarative language, meaning that it tells the SQL engine what to do, not how. This is in contrast to an imperative language such as C, in which how to do something is clearly laid out.
This means that not all statements will execute as expected. Of particular note are boolean expressions, which may not evaluate from left-to-right as written. For example, the following code is not guaranteed to execute without a divide by zero error:
SELECT 'null' WHERE 1 = 1 OR 1 / 0 = 0
The reason for this is the query optimizer chooses the best (most efficient) way to execute a statement. This means that, for example, a value may be loaded and filtered before a transforming predicate is applied, causing an error. See the second link above for an example
See: here and here.

Answer 5

"Order of execution" is probably a bad mental model for SQL queries. Its hard to actually write a single query that would actually depend on order of execution (this is a good thing). Instead you should think of all join and where clauses happening simultaneously (almost like a template)
That said you could run display the Execution Plans which should give you insight into it.
However since its's not clear why you want to know the order of execution, I'm guessing your trying to get a mental model for this query so you can fix it in some way. This is how I would "translate" your query, although I've done well with this kind of analysis there's some grey area with how precise it is.
FROM AND WHERE CLAUSE
Give me all the Event Participants rows. from [3rdi_EventParticipants
Also give me all the Event Signup rows that match the Event Participants rows on SignUpID inner join 3rdi_EventSignup] as ES on EP.SignUpId = ES.SignUpId
But Only for Event 13 EP.EventId = 13
And only if the user id has a record in the user roles table where the role id is not in 1,2,19,20,21,22
userid in (
select distinct userid from userroles
--where roleid not in(6,7,61,64) and roleid not in(1,2))
where roleid not in(19, 20, 21, 22) and roleid not in(1,2))
SELECT CLAUSE
For each of the rows give me a unique ID
Row_number() OVER(ORDER BY (SELECT 1)) AS 'Serial Number',
The participants First Name EP.FirstName
The participants Last Name Ep.LastName
The Booking Role name GetBookingRoleName
Go look in the Event Dates and find out what the first eventDate where the EventId = 13 that you find
(select top 1 convert(varchar(10),eventDate,103)from [3rdi_EventDates] where EventId=13) as EventDate
Finally translate the GetBookingRoleName in Category. I don't have a table for this so I'll map it manually (CASE [dbo].[GetBookingRoleName](ES.UserId,EP.BookingRole)
WHEN '90 Day Client' THEN 'DC'
WHEN 'Association Client' THEN 'DC'
WHEN 'Autism Whisperer' THEN 'DC'
WHEN 'CampII' THEN 'AD'
WHEN 'Captain' THEN 'AD'
WHEN 'Chiropractic Assistant' THEN 'AD'
WHEN 'Coaches' THEN 'AD'
END) as Category
So a couple of notes here. You're not ordering by anything when you select TOP. You should probably have na order by there. You could also just as easily put that in your from clause e.g.
from [3rdi_EventParticipants] as EP
inner join [3rdi_EventSignup] as ES on EP.SignUpId = ES.SignUpId,
(select top 1 convert(varchar(10),eventDate,103)
from [3rdi_EventDates] where EventId=13
Order by eventDate) dates

Answer 6

There is a logical order to evaluation of the query text, but the database engine can choose what order execute the query components based upon what is most optimal. The logical text parsing ordering is listed below. That is, for example, why you can't use an alias from SELECT clause in a WHERE clause. As far as the query parsing process is concerned, the alias doesn't exist yet.
FROM
ON
OUTER
WHERE
GROUP BY
CUBE | ROLLUP
HAVING
SELECT
DISTINCT
ORDER BY
TOP
See the Microsoft documentation (see "Logical Processing Order of the SELECT statement") for more information on this.

Answer 7

Simplified order for T-SQL -> SELECT statement:
1) FROM
2) Cartesian product
3) ON
4) Outer rows
5) WHERE
6) GROUP BY
7) HAVING
8) SELECT
9) Evaluation phase in SELECT
10) DISTINCT
11) ORDER BY
12) TOP
as I had done so far - same order was applicable in SQLite.
Source => SELECT (Transact-SQL)
... of course there are (rare) exceptions.