SQL Server中SELECT (Transact-SQL)语法定义和解释

语法定义：

<SELECT statement> ::=[ WITH { [ XMLNAMESPACES , ] [ <common_table_expression> [ , ...n ] ] } ]<query_expression>[ ORDER BY <order_by_expression> ][ <FOR Clause> ][ OPTION ( <query_hint> [ , ...n ] ) ]
<query_expression> ::={ <query_specification> | ( <query_expression> ) }[  { UNION [ ALL ] | EXCEPT | INTERSECT }<query_specification> | ( <query_expression> ) [ ...n ] ]
<query_specification> ::=
SELECT [ ALL | DISTINCT ][ TOP ( expression ) [ PERCENT ] [ WITH TIES ] ]<select_list>[ INTO new_table ][ FROM { <table_source> } [ , ...n ] ][ WHERE <search_condition> ][ <GROUP BY> ][ HAVING <search_condition> ]
[ ; ]

解释：

1. <SELECT statement>、<query_expression> 、<query_specification>等，这些用尖括号括起来的，称为术语
2. ::=后面跟随的是对术语的解释
3. 像这种被中括号括起来的[ WHERE <search_condition> ] [ <GROUP BY> ] [ HAVING <search_condition> ]都表示为可选元素
4. {}被这种花括号括起来的表示必选元素，如果花括号中有|这种符号，表示只能选其一，比如这种： { UNION [ ALL ] | EXCEPT | INTERSECT }
5. [ ...n ] 表示对前面所写元素的0到多次重复
6. [ , ...n ] 表示对前面所写元素的0到多次重复，用逗号隔开

把上面Select语法拆分成一个最简单的查询语句
上面Select语法定义是由3个术语组成，分别是：

<query_specification> ::=
SELECT [ ALL | DISTINCT ][ TOP ( expression ) [ PERCENT ] [ WITH TIES ] ]<select_list>[ INTO new_table ][ FROM { <table_source> } [ , ...n ] ][ WHERE <search_condition> ][ <GROUP BY> ][ HAVING <search_condition> ]
[ ; ]

<query_expression> ::={ <query_specification> | ( <query_expression> ) }[  { UNION [ ALL ] | EXCEPT | INTERSECT }<query_specification> | ( <query_expression> ) [ ...n ] ]

<SELECT statement> ::=[ WITH { [ XMLNAMESPACES , ] [ <common_table_expression> [ , ...n ] ] } ]<query_expression>[ ORDER BY <order_by_expression> ][ <FOR Clause> ][ OPTION ( <query_hint> [ , ...n ] ) ]

1. 对<query_specification> 进行简化拆解，就只剩

SELECT  <select_list>

2. 对<query_expression> 进行简化拆解，就只剩 { <query_specification> | ( <query_expression> ) }然后选择一个，我们把<query_specification>带入就是

SELECT  <select_list>

3. 对<SELECT statement>
   进行简化拆解，就只剩<query_expression>，然后继续带入，最简查询语句就出来了

SELECT  <select_list>

由上面定义我们知道 <select_list>也是术语，他可以是一个表达式，一个值，等，或者是通过逗号分隔出来的各种表达式。