\nSQL Server 2008 adds support for the 'let' clause in FLWOR expressions. The for and let clauses have a similar purpose, to bind content (tuples) to variables. Either one can begin a FLWOR expression:\n<\/p>\n
\ndeclare @x xml = '';
\nselect @x.query('
\nfor $i in (1,2,3)
\nreturn $i
\n');
\n> returns 1 2 3\n<\/p>\n
\ndeclare @x xml = '';
\nselect @x.query('
\nlet $i := (1,2,3)
\nreturn $i
\n');
\n> returns 1 2 3\n<\/p>\n
\nThe distinction is that let is an assignment clause, in the simple statement using 'let' above, $i refers to the entire sequence (1,2,3). The for clause sets up an iterator. The simple statement above using 'for' loops 3 times and each time through the loop $i refers to a single member of the sequence. So, if I add an 'order by' clause, the results are quite different.\n<\/p>\n
\ndeclare @x xml = '';
\nselect @x.query('
\nfor $i in (1,2,3)
\norder by $i descending
\nreturn $i
\n');
\n> returns 3 2 1\n<\/p>\n
\ndeclare @x xml = '';
\nselect @x.query('
\nlet $i := (1,2,3)
\norder by $i descending
\nreturn $i
\n');
\n> error:
\n> XQuery [query()]: 'order by' requires a singleton (or empty sequence), found operand of type 'xs:integer +'\n<\/p>\n
\nOne limitation on the XQuery let clause is that it does not support constructed elements. So this statement works fine:\n<\/p>\n
\ndeclare @x xml = '';
\nselect @x.query('
\nlet $x := 1
\nreturn $x
\n');
\n> returns 1\n<\/p>\n
\nbut this statement does not:\n<\/p>\n
\ndeclare @x xml = '';
\nselect @x.query('
\nlet $x := ( <foo>2<\/foo>, <bar>2<\/bar> )
\nreturn $x
\n');
\n> error:
\nXQuery [query()]: 'let' is not supported with constructed XML\n<\/p>\n
\nSo Let the use of the let clause begin…no longer do I have to explain what a "FWOR" expression is, hooray. However, nota bene. When 'let' is used inside a loop, it's evaluated each time around the loop:\n<\/p>\n
\ndeclare @x xml = '';
\nselect @x.query('
\nfor $i in (1,2,3)
\nlet $j := 42
\nreturn ($i, $j)
\n');
\n> returns 1 42 2 42 3 42
\n> $j is evaluated three times\n<\/p>\n
\n <\/p>\n","protected":false},"excerpt":{"rendered":"
SQL Server 2008 adds support for the 'let' clause in FLWOR expressions. The for and let clauses have a similar purpose, to bind content (tuples) to variables. Either one can begin a FLWOR expression: declare @x xml = ''; select @x.query(' for $i in (1,2,3) return $i '); > returns 1 2 3 declare @x […]<\/p>\n","protected":false},"author":7,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[29,37],"tags":[],"class_list":["post-744","post","type-post","status-publish","format-standard","hentry","category-sql-server-2008","category-sql-server-xml"],"yoast_head":"\n