MySQL UNION中语句评估和变量赋值的顺序

Question

问题：在下面的UNIONized查询中，如何强制@col在对依赖派生查询进行求值之前分配？要求：它需要在一个查询中完成。

CREATE TABLE tbl (col CHAR(1) NOT NULL UNIQUE); 

INSERT INTO tbl (col) VALUES ('a'), ('b'), ('c'), ('d'), ('e'), ...; 

-- Now, for some value of "col", fetch that record and the 
-- immediately preceding and following records as ordered by "col" 
-- 
-- If you care to test this, be sure to SET @col := NULL before 
-- subsequent executions to simulate a "fresh" MySQL session... 
-- 
SELECT @col := col AS col -- Fetch particular record given a value of 
    FROM tbl     -- "col". 
WHERE col = 'd' 
UNION ALL 
SELECT col     -- Fetch the immediately preceding record, 
    FROM ( SELECT col   -- ordered by "col" 
      FROM tbl 
      WHERE col < @col 
     ORDER BY col DESC 
      LIMIT 1) preceding 
UNION ALL 
SELECT col     -- Fetch the immediately following record, 
    FROM ( SELECT col   -- ordered by "col" 
      FROM tbl 
      WHERE col > @col 
     ORDER BY col ASC 
      LIMIT 1) following 
ORDER BY col ASC; 

背景：从工会组织上面的查询我有望获得三个记录：记录匹配的精确和独特的“关口”值将紧随前面和后面的记录的“关口”的命令。

但是，第一个运行的查询只产生一条记录，一条匹配用户提供的“col”值。随后的运行给了我期望的三个。我的推论是@col直到衍生查询preceding和following被评估后才被分配 - 这不是我所期望的从左到右的从上到下的评估顺序。

（我正在试图回答细化到this question，但跑在这个困难。）

Answer 1

不要UNION @col的分配与其他查询。

有一个查询为@col赋值，还有一个单独的查询将该记录包含在结果中。

SELECT @col := col AS col -- Fetch particular record given a value of 
    FROM tbl     -- "col", assigning the identifier to @col. 
WHERE col = 'd' 



SELECT col     -- Now include the above record in the 
    FROM tbl     -- Final result-set 
WHERE col = @col 

UNION ALL 

SELECT col     -- Fetch the immediately preceding record, 
    FROM ( SELECT col   -- ordered by "col" 
      FROM tbl 
      WHERE col < @col 
     ORDER BY col DESC 
      LIMIT 1) preceding 

UNION ALL 

SELECT col     -- Fetch the immediately following record, 
    FROM ( SELECT col   -- ordered by "col" 
      FROM tbl 
      WHERE col > @col 
     ORDER BY col ASC 
      LIMIT 1) following 
ORDER BY col ASC; 

Answer 2

SELECT @col := col AS col -- Fetch particular record given a value of 
    FROM tbl     -- "col". 
WHERE col = 'd' 
UNION ALL 
SELECT col     -- Fetch the immediately preceding record, 
    FROM 

(@colx:=col), -- Assigne it here 

     ( SELECT col   -- ordered by "col" 
      FROM tbl 
      WHERE col < @colx 
     ORDER BY col DESC 
      LIMIT 1) preceding 
UNION ALL 
SELECT col     -- Fetch the immediately following record, 
    FROM 

(@colx:=col), -- Assign it here also 

     ( SELECT col   -- ordered by "col" 
      FROM tbl 
      WHERE col > @colx 
     ORDER BY col ASC 
      LIMIT 1) following 
ORDER BY col ASC;