Order of execution in a composite application and double ampersand

Hello

I had a doubt as to the order of execution of a compound query. As a general rule, I read his right to the left, up and down.
I understand that once the value has been substituted to the variable must be explicitly undefined and all queries are working well.


I'm confused by running the example below.

1. when I run this example below that I've been invited twice .
I guess once the results and which is assigned a value & & who should himself search for new value

Select the function
employees join jobs (job_id) using
where employee_id = & who
Union
Select the function
of job_history join jobs (job_id) using
where employee_id = & & who;


2. here in the example below, the value is currently not invited only once .
. Is the lowest first executed query and if so why? Is there a special case in which this happens?


Select the function
employees join jobs (job_id) using
where employee_id = & & who
Union
Select the function
of job_history join jobs (job_id) using
where employee_id = & who;


the example below works fine with a prompt for value. Performs one of the *'& &' work as a single & *.

Select the function
employees join jobs (job_id) using
where employee_id = & & who
Union
Select the function
of job_history join jobs (job_id) using
where employee_id = & & who;



Help, please
Thank you
Jayshree

Hi, Mary,

to_learn wrote:
Hello

I had a doubt as to the order of execution of a compound query. As a general rule, I read his right to the left, up and down.

Enforcement order has nothing to do with this problem.
The substitution variables are resolved by SQL * more as it analyzes your order. I think he analyzes in order as it appears in your script, or on the command line as you type. Turn on "SET CHECK" to verify this.
After SQL * more analysis of the command has completed, it passes the command (with replaced by their values for substitution variables) for the back-end, which compiles, and then only, executes the statement.

I understand that once the value has been substituted to the variable must be explicitly undefined and all queries are working well.

I'm confused by running the example below.

1. when I run this example below that I've been invited twice .
I guess the results & who is assigned a value once & who should himself search for new value

It's true.
Before you run the command, & which is not defined.

Select the function
employees join jobs (job_id) using
where employee_id = & which

Since & that is not defined, SQL * Plus asks you to give a value to it.
After the analysis of this line, & who is still undefined, because you used a single &.

Union
Select the function
of job_history join jobs (job_id) using
where employee_id = & &whom;

Since & that is not defined, SQL * Plus asks you to give a value to it.
After the analysis of this line, & which is defined, because you used a double &. (Use the command SET to verify this.)

2. here in the example below, the value is currently not invited only once .
. Is the lowest first executed query and if so why? Is there a special case in which this happens?

Yes, once, before running the script, & is not defined yet.

Select the function
employees join jobs (job_id) using
where employee_id = & who

Since & that is not defined, SQL * Plus asks you to give a value to it.
After the analysis of this line, & which is defined, because you used a double &.

Union
Select the function
of job_history join jobs (job_id) using
where employee_id = &whom;

Since & that IS defined, SQL * Plus, do not stop and ask you to provide a value for it.

the example below works fine with a prompt for value. Performs one of the *'& ' work as a single & *.

Yes; but I don't think it's a very useful way to think about. In fact, two of them work as single & s one way or another.
Once again, & which is not defined at this stage.

Select the function
employees join jobs (job_id) using
where employee_id = & who

Since & that is not defined, SQL * Plus asks you to give a value to it.
After the analysis of this line, & which is defined, because you used a double &.
Using a single & cause also stop and ask. If the variable is not set, SQL * more stops and you request a value, regardless of whether you have used a single & double or &.

Union
Select the function
of job_history join jobs (job_id) using
where employee_id = & &whom;

Since & that set, SQL * Plus you do not give a value for it.
Using a single & would also result in the same behavior. If the variable is already defined, SQL * Plus, do not stop and you ask a value; It uses the previously given value, regardless of if you have used a single & double or &.

Tags: Database

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