I had a discussion with a teammate on the topic whether the terms clause and expression can be used interchangeably. For example, is it correct/common to call a variable that stands for an expression a=b (e.g. that participates in a statement SELECT * WHERE expression) a clause?
Edit
It would be useful is someone could give precise definitions of what clause, expression and statement are in SQL world.
In SQL Terms, "clause" is usually used to refer to a section of a statement, usually introduced by the keyword it's named after - e.g. a typical SELECT statement would be composed of a SELECT clause, a FROM clause and a WHERE clause. Within the FROM clause, some people may refer to JOIN clauses and ON clauses. However, this is by no means 100% accepted usage.
When it comes to "statement" and "expression", it's fairly standard usage - an expression is something that produces a value. In most languages, this is understood, further, to be something that produces a scalar value. In SQL, this is slightly modified because when you encounter an expression when working with a row set, the expression will produce one scalar value per row (or per group or partition, if grouping or partitioning are involved and it's in the relevant location).
Finally, a statement is a complete "something" that your database engine can understand and produce results for. It doesn't produce a value but it may produce a result set. You can't just send a FROM clause to the database - it has to be part of a larger statement, such as the SELECT statement I mentioned in my first paragraph.
The answer is NO, expression evaluates to something may be a boolean value or string or number where as a clause forms a rule for the data to satisfy and only then the record forms part of the result.
select * from TABLE where /*clause 1*/ field1 = field2
and /*clause 2*/field3 = /*expression*/ field1 + field2
In the above select statement
first clause forms a rule which is field1 should be equal to field2
Second clause form a rule which is field3 should be equal to the result of the > expression field1 + field2
UPDATE
There are various clauses in SQL like from, where, order by, group by and having. from clause tells from which table to read and order by tells how to arrange the result. Clauses control from where data to be read, what data be formed as part of the select statement and how the data to be presented.
Expression on the other hand evaluate to a value of some datatype.
A Statement, is a structured query build with the clauses.
Related
While watching Troy Hunt's fantastic course on SQLi, I've noticed that he ends up using this strategy to see if a table has a specific column:
select * from TableA order by (select top 1 some_column from TableB) desc
This expression will get executed by SQL Server, but what will it do for the order by clause? I've seen expressions being used with order by before (case when then else end), but I'm really curious to understand how SQL can process the previous query without any errors...
EDIT: Giving more info because it seems like my initial post was not clear enough:
I know this is not the best strategy for getting table or column name though SQLi (that's not what I'm asking)
I'm not interested in knowing how to protect against this (I know how to do that already)
I know that sorting by a constant value doesn't make sense (though it allows you to run these types of "boolean queries")
What t I really want to know is why it works.
So, going back to the docs, the order by clause expects an order_by_expression, which is described as:
Specifies a column or expression on which to sort the query result set. A sort column can be specified as a name or column alias, or a nonnegative integer representing the position of the column in the select list.
According to the docs, an expression is:
Is a combination of symbols and operators that the SQL Server Database Engine evaluates to obtain a single data value. Simple expressions can be a single constant, variable, column, or scalar function. Operators can be used to join two or more simple expressions into a complex expression.
As #SMor demonstrated, the query does run if you replace the order by select expression with a simple select 'A':
select * from TableA order by (select 'A') desc
But this does not work:
select * from TableA order by 'A' desc
So, the question is: why is select 'A' accepted by SQL Server in the order by clause? Doesn't it produce a constant too? Since a constant is an expression and taking into account the definition for the order by clause, shouldn't it thrown an error in both cases?
Thanks.
The use of (select top 1 some_column from TableB) is an example of a scalar subquery. This is a subquery that returns exactly one column and at most one row. It can be used anywhere a literal value can be used -- and perhaps in some other places as well. Apparently, it can be used in an order by, even though SQL Server does not allow a literal value for order by.
The most common type of scalar subquery is a correlated subquery, which has a where clause that connects the subquery to the outer query. This is not an example of a scalar subquery.
In fact, this is not an example of anything useful as far as I can tell. It has one major shortcoming, which is the use of top without order by. The value returned by the subquery is indeterminate. That seems like a bad practice, and particularly bad if you are trying to teach people SQL.
And, it is probably going to be evaluated once. So the subquery would return a constant value and would not contribute much to a meaningful ordering.
Are Postgres SELECT DISTINCT queries deterministic?
Will SELECT DISTINCT somecolumn FROM sometable return the same result (including order) if the table (and entire database) goes unchanged?
In the Select Query Documentation the Description section notes:
If the ORDER BY clause is specified, the returned rows are sorted in the specified order. If ORDER BY is not given, the rows are returned in whatever order the system finds fastest to produce.
In the DISTINCT ON clause section they add:
Note that the "first row" of each set is unpredictable unless ORDER BY is used to ensure that the desired row appears first.
Generally, is this still true when the database goes un-changed?
This answer assumes that the expressions in the select are deterministic. Otherwise, the question seems trivial.
The ordering is not specified, so it could change between runs of the query -- or on a different system. However, the result set should be the same.
Your second quote from the documentation is for distinct on. That is not-deterministic, unless you are using a stable sort.
Note: You might get non-deterministic results if you are using a case-insensitive collation. The built-in collations are case-sensitive; and case insensitivity means that the original expressions are not deterministic.
My professor is teaching sql 2, and used an statement like the code below into a query:
HAVING SUM(column) > subselect
Where subselect is something like SELECT AVG(column) FROM ...
This subselect returns only one value, but I could not understand how is it possible to compare a function (the sum) with a subselect. The subselect should return a table, right? Then how is it possible to compare a table with a value? That did not make sense to me.
Thanks in advance.
SQL has a concept of scalar subqueries. These are subqueries that return exactly one column and at most one row. Scalar subqueries can be used in almost all cases where a single value ("scalar") can be used.
If the scalar subquery returns no rows, then the value is treated as NULL.
(I should add that some databases support tuples. A tuple is a set of scalar values that is treated as a single value. In such databases "scalar" subqueries can return more than one value, but these are converted to a tuple. This is not relevant to the question being asked; tuples are just another example of a "single" value.)
In principle you are right, if you look at it in a relational way.
But SQL is an industry standard and allows the abbreviation of comparing a scalar value with a result table with only a single row and column.
Depending on the exact implementation, it even allows to compare a scalar with a list of values (a column with more than one value), although you actually should write ... value > ALL (subselect), it is often accepted without the ALL keyword.
This is both valid syntax in the where and in the having clause
Assume value is an int and the following query is valid:
SELECT blah
FROM table
WHERE attribute = value
Though MAX(expression) returns int, the following is not valid:
SELECT blah
FROM table
WHERE attribute = MAX(expression)
OF course the desired effect can be achieved using a subquery, but my question is why was SQL designed this way - is there some reason why this sort of thing is not allowed? Students coming from programming languages where you can always replace a data-type by a function call that returns that type find this issue confusing. Is there an explanation one can give them rather than just saying "that's the way it is"?
It's just because of the order of operations of a query.
FROM clause
WHERE clause
GROUP BY clause
HAVING clause
SELECT clause
ORDER BY clause
WHERE just filters the rows returned by FROM. An aggregate function like MAX() can't have a result returned because it hasn't even been applied to anything.
That's also the reason, why you can't use aliases defined in the SELECT clause in a WHERE clause, but you can use aliases defined in FROM clause.
A where clause checks every row to see if it matches the conditions specified.
A max computes a single value from a row set. If you put a max, or any other aggregate function into a where clause, how can SQL server figure out what rows the max function can use until the where clause has finished it filter?
This deals with the order that SQL Server processes commands in. It runs the WHERE clause before a GROUP BY or any aggregate. Since a where clause runs first, SQL Server can't tell if a row will be included in an aggregate until it processes the where. That is what the HAVING clause is for. HAVING runs after the GROUP BY and the WHERE and can include MAX since you have already filtered out the rows you don't want to use. See http://www.bennadel.com/blog/70-SQL-Query-Order-of-Operations.htm for a good explanation of the order in which SQL commands run.
Maybe this work
SELECT blah
FROM table
WHERE attribute = (SELECT MAX(expresion) FROM table1)
The WHERE clause is specifically designed to test conditions against raw data (individual rows of the table). However, MAX is an aggregate function over multiple rows of data. Basically, without a sub-select, the WHERE clause knows nothing about any rows in the table except for the current row. So how can you determine the maximum value over a whole bunch of rows when you don't even know what those rows are?
Yes, it's a little bit of a simplification, especially when dealing with joins, but the same principle applies. WHERE is always row-by-row, so that's all it really knows about.
Even if you have a GROUP BY clause, the WHERE clause still only processes one row at a time in the raw data before grouping. It doesn't know the value of a column in any other rows, so it has no way of knowing which row has the maximum value.
Assuming this is MS SQL Server, the following would work.
SELECT TOP 1 blah
FROM table
ORDER BY expression DESC
Is the following possible according to standard(!) SQL?
What minimal changes should be neccessary in order to be conforming to the standard (if it wasn't already)?
It works as expected in MySQL, iff the first row has the maximum value for NumberOfPages.
SELECT *
FROM Book
HAVING NumberOfPages = MAX(NumberOfPages)
The following is written in the standard:
HAVING <search condition>
Let G be the set consisting of every column referenced by a <column reference> contained in the <group by clause>.
Each column reference directly contained in the <search condition> shall be one of the following:
An unambiguous reference to a column that is functionally dependent on G.
An outer reference.
source
Can somebody explain me, why it should be possible according to the standard?
In MySQL, it perfectly works.
Despite the Mimer Validator result, I don't believe yours is valid Standard SQL.
A HAVING clause without a GROUP BY clause is valid and (arguably) useful syntax in Standard SQL. Because it operates on the table expression all-at-once as a set, so to speak, it only really makes sense to use aggregate functions. In your example:
Book HAVING NumberOfPages = MAX(NumberOfPages)
is not valid because when considering the whole table, which row does NumberOfPages refer to? Likewise, it only makes sense to use literal values in the SELECT clause.
Consider this example, which is valid Standard SQL:
SELECT 'T' AS result
FROM Book
HAVING MIN(NumberOfPages) < MAX(NumberOfPages);
Despite the absence of the DISTINCT keyword, the query will never return more than one row. If the HAVING clause is satisfied then the result will be a single row with a single column containing the value 'T' (indicating we have books with differing numbers of pages), otherwise the result will be the empty set i.e. zero rows with a single column.
I think the reason why the query does not error in mySQL is due to propritary extensions that cause the HAVING clause to (logically) come into existence after the SELECT clause (the Standard behaviour is the other way around), coupled with the implicit GROUP BY clause mentioned in other answers.
“When GROUP BY is not used, HAVING behaves like a WHERE clause.”
The difference between where and having: WHERE filters ROWS while HAVING filters groups
SELECT SUM(spending) as totSpending
FROM militaryspending
HAVING SUM(spending) > 200000;
Result
totSpending
1699154.3
More detail, please consult
https://dba.stackexchange.com/questions/57445/use-of-having-without-group-by-in-sql-queries/57453
From the standard (bold added from emphasis)
1) Let HC be the having clause. Let TE be the table expression that immediately contains HC. If TE does not immediately contain a group by clause, then “GROUP BY ()” is implicit. Let T be the
descriptor of the table defined by the GBC immediately contained in TE and let R be
the result of GBC.
With the implicit group by clause, the outer reference can access the TE columns.
However, the certification to these standards is very much a self-certification these days, and the example you gave would not work across all of the main RDBMS providers.
Yes We can write the SQL query without Group by but write the aggregate function
in our query.
select sum(Salary) from ibs having max(Salary)>1000