Sql server 规划WHERE子句。@stakx我刚刚添加了一些附加信息（在答案的末尾），感谢这个例子，并确认DISTINCT确实会使所有三个查询返回相同的结果。关于为什么查询优化器不更智能，我故意避免这样问这个问题，因为答案应该是“因为它不是”。（正如Eric Lipp

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规划

WHERE

子句。@stakx我刚刚添加了一些附加信息（在答案的末尾），感谢这个例子，并确认

DISTINCT

确实会使所有三个查询返回相同的结果。关于为什么查询优化器不更智能，我故意避免这样问这个问题，因为答案应该是“因为它不是”。（正如Eric Lippert喜欢指出的关于C语言，特性不是默认实现的，也就是说，除非经过深思熟虑来实现它们。）我想换个角度问会更有价值，也就是说，如果有理由证明这种行为是有道理的。

CREATE TABLE y (Id  INT NOT NULL PRIMARY KEY      );
CREATE TABLE x (YId INT NOT NULL REFERENCES y (Id));
DECLARE @YId INT = …;

/* 1: */  SELECT x.* FROM x                              WHERE x.YId = @YId;
/* 2: */  SELECT x.* FROM x INNER JOIN y ON                    x.YId = @YId;
/* 3: */  SELECT x.* FROM x INNER JOIN y ON x.YId = y.Id WHERE y.Id  = @YId;

<joined_table> ::= 
{
    <table_source> <join_type> <table_source> ON <search_condition> 
    | <table_source> CROSS JOIN <table_source> 
    | left_table_source { CROSS | OUTER } APPLY right_table_source 
    | [ ( ] <joined_table> [ ) ] 
}
<join_type> ::= 
    [ { INNER | { { LEFT | RIGHT | FULL } [ OUTER ] } } [ <join_hint> ] ]
    JOIN

SELECT
  U.UserName, P.PrivilegeName, P.Status
FROM
 dbo.Users U
LEFT JOIN dbo.Privileges P
  ON U.UserID = P.UserID AND P.PrivilegeName = 'EditComment'

SELECT
  U.UserName, P.PrivilegeName, P.Status
FROM
 dbo.Users U
LEFT JOIN (SELECT UserID, Status FROM dbo.Privileges WHERE P.PrivilegeName = 'EditComment') P
  ON U.UserID = P.UserID

SELECT
  U.UserName, P.PrivilegeName, P.Status
FROM
 dbo.Users U
LEFT JOIN dbo.Privileges P
  ON U.UserID = P.UserID
WHERE
  P.PrivilegeName = 'EditComment'

/* 4: */ SELECT x.* FROM x INNER JOIN y ON                    1=1;
/* 5: */ SELECT x.* FROM x CROSS JOIN y

/* 6: */ SELECT x.* FROM x CROSS JOIN y WHERE x.YId = @YId

2 / SELECT x.* FROM x INNER JOIN y ON                    x.YId = @YId

select
    tx.*
from
    (
        select x.* from x where x.YId = @YId
    ) tx
    cross join y

declare @ty table (Id int not null primary key)
declare @tx table (Id Char(1) not null primary key, yId int not null)

insert into @ty values (1) ,(2), (3)
insert into @tx values ('A', 1), ('B', 1), ('C', 2)

declare @YId int = 1

SELECT x.* FROM @tx x                             WHERE x.YId = @YId
SELECT x.* FROM @tx x INNER JOIN @ty y ON   x.yId = @YId
SELECT x.* FROM @tx x INNER JOIN @ty y ON x.YId = y.Id WHERE y.Id  = @YId

A   1
B   1

A   1
A   1
A   1
B   1
B   1
B   1

A   1
B   1

insert into y values(1),(2);
insert into x values(1),(2);

YID
1
1