What Does "Where 1" Mean in Sql

What does WHERE 1 mean in SQL?

It doesn't. It means ALWAYS TRUE so it won't have any filtering impact on your query. Query planner will probably ignore that clause.

It's usually used when you build a client side query by concatenating filtering conditions.

So, if your base query is stored in a string like this (example is in PHP, but it certainly applies to many other languages):

$sql = "select * from foo where 1 ";

Then you can just concatenate a lot of filtering conditions with an AND clause regardless of it being the first condition you are using or not:

// pseudo php follows...
if ($filter_by_name) {
    $sql = $sql . " and name = ? ";
}
if ($filter_by_number) {
    $sql = $sql . " and number = ? ";
}
// so on, and so forth.

What is the purpose of using WHERE 1=1 in SQL statements?

Yeah, it's typically because it starts out as 'where 1 = 0', to force the statement to fail.

It's a more naive way of wrapping it up in a transaction and not committing it at the end, to test your query. (This is the preferred method).

why would you use WHERE 1=0 statement in SQL?

A query like this can be used to ping the database. The clause:

WHERE 1=0

Ensures that non data is sent back, so no CPU charge, no Network traffic or other resource consumption.

A query like that can test for:

• server availability
• CUST_ATTR49 table existence
• ID column existence
• Keeping a connection alive
• Cause a trigger to fire without changing any rows (with the where clause, but not in a select query)
• manage many OR conditions in dynamic queries (e.g WHERE 1=0 OR <condition>)

Why would someone use WHERE 1=1 AND conditions in a SQL clause?

If the list of conditions is not known at compile time and is instead built at run time, you don't have to worry about whether you have one or more than one condition. You can generate them all like:

and <condition>

and concatenate them all together. With the 1=1 at the start, the initial and has something to associate with.

I've never seen this used for any kind of injection protection, as you say it doesn't seem like it would help much. I have seen it used as an implementation convenience. The SQL query engine will end up ignoring the 1=1 so it should have no performance impact.

What does it mean by select 1 from table?

SELECT 1 FROM TABLE_NAME means, "Return 1 from the table". It is pretty unremarkable on its own, so normally it will be used with WHERE and often EXISTS (as @gbn notes, this is not necessarily best practice, it is, however, common enough to be noted, even if it isn't really meaningful (that said, I will use it because others use it and it is "more obvious" immediately. Of course, that might be a viscous chicken vs. egg issue, but I don't generally dwell)).

 SELECT * FROM TABLE1 T1 WHERE EXISTS (
     SELECT 1 FROM TABLE2 T2 WHERE T1.ID= T2.ID
 );

Basically, the above will return everything from table 1 which has a corresponding ID from table 2. (This is a contrived example, obviously, but I believe it conveys the idea. Personally, I would probably do the above as SELECT * FROM TABLE1 T1 WHERE ID IN (SELECT ID FROM TABLE2); as I view that as FAR more explicit to the reader unless there were a circumstantially compelling reason not to).

EDIT

There actually is one case which I forgot about until just now. In the case where you are trying to determine existence of a value in the database from an outside language, sometimes SELECT 1 FROM TABLE_NAME will be used. This does not offer significant benefit over selecting an individual column, but, depending on implementation, it may offer substantial gains over doing a SELECT *, simply because it is often the case that the more columns that the DB returns to a language, the larger the data structure, which in turn mean that more time will be taken.

Why would a sql query have where 1 = 1

Was it dynamic queries? Sometimes that's helpful when building dynamic queries based on parameters that are optional.

What does TOP 1 mean in an sql query?

The query in the example will return the first RequestID from the table PublisherRequests.

The order of the results without an Order By clause is arbitrary. So, your example will return an arbitrary RequestID (i.e. the first RequestID in an arbitrarily ordered list of RequestIDs).

You can change the order by defining an Order By.

For example, to get the last entered ID, you can write

Select Top 1 RequestID
From PublisherRequests
Order By RequestID Desc

Updated to include corrected order information from @Kirtan Gor and @AlexK

What does SQL clause GROUP BY 1 mean?

It means to group by the first column of your result set regardless of what it's called. You can do the same with ORDER BY.

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