Question

问题：在下面的UNIONized查询中，如何在评估依赖派生查询之前强制@col分配？要求：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：从上面的UNIONized查询中，我希望获得三条记录：匹配精确且唯一的“col”值的记录，以及“col”排序的前一条和后一条记录。

但是，查询的第一次运行只产生一条记录，一条记录与用户提供的“col”值匹配。后续的跑步给了我期待的三个。我的推论是，在评估派生查询@col和preceding之后才会分配following - 这不是从左到右从上到下的评估顺序I' d预期。

（我试图改进this question的答案，但遇到了这个困难。）

Answer 1

不要将@col的分配与其他查询联系起来。

有一个查询可以为@col分配值，还有一个单独的查询可以在结果中包含该记录。

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;

MySQL UNION中的语句评估和变量赋值顺序

2 个答案: