MySQL select...into variable 产生 null，即使数据在那里

Question

我的存储过程执行SELECT ... INTO var产生 NULL，但如果我SELECT自己重复相同的操作，我会得到一个值。

这是架构的相关部分

CREATE TABLE UrlAuthority
(
     id              BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY
    ,name            VARCHAR(255) NOT NULL COMMENT 'includes TLD suffix'
    ,UNIQUE(name)
)
ENGINE = INNODB DEFAULT CHARACTER SET utf8 COLLATE utf8_unicode_ci;



CREATE TABLE UrlFqdn
(
     id              BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY
    ,authority       BIGINT NOT NULL COMMENT 'references example.com'
    ,name            VARCHAR(255) NOT NULL COMMENT 'host.example.com'
    ,FOREIGN KEY (authority) REFERENCES UrlAuthority (id)
    ,UNIQUE (authority, name)
)
ENGINE = INNODB DEFAULT CHARACTER SET utf8 COLLATE utf8_unicode_ci;



CREATE TABLE Url
(
     id              BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY
    ,fqdn            BIGINT NOT NULL
    ,path            VARCHAR(255) NOT NULL
    ,FOREIGN KEY (fqdn) REFERENCES UrlFqdn (id)
    ,UNIQUE (fqdn, path)
)
ENGINE = INNODB DEFAULT CHARACTER SET utf8 COLLATE utf8_unicode_ci;

这是程序

CREATE PROCEDURE UrlToId (url TEXT)
BEGIN
    DECLARE host      TEXT;
    DECLARE authority TEXT;
    DECLARE tld       TEXT;
    DECLARE pathtext  TEXT;

    DECLARE authority_tld TEXT;
    DECLARE fqdn          TEXT;
    DECLARE authority_id  BIGINT;
    DECLARE fqdn_id       BIGINT;
    DECLARE url_id        BIGINT;
    DECLARE err           TEXT;

    CALL UnpackURL (url, host, authority, tld, pathtext);

    SELECT JoinHostNames(authority, tld)      INTO authority_tld;
    SELECT JoinHostNames(host, authority_tld) INTO fqdn;

    INSERT IGNORE INTO UrlAuthority (name) VALUES (authority_tld);

    SELECT id
    INTO authority_id
    FROM UrlAuthority
    WHERE name=authority_tld;

    -- LOG ('UrlAuthority', authority_tld, authority_id)

    INSERT IGNORE INTO UrlFqdn (authority, name)
    VALUES (authority_id, fqdn);

    SELECT id
    INTO fqdn_id
    FROM UrlFqdn
    WHERE authority=authority_id AND name=fqdn;

    -- LOG ('UrlFqdn', fqdn, fqdn_id)

    INSERT IGNORE INTO Url (fqdn, path) VALUES (fqdn_id, pathtext);

    -- LOG ('Url', fqdn_id, pathtext)

    SELECT id FROM Url WHERE fqdn=fqdn_id AND path=pathtext;
END
|

这些LOG行有点元编程，它们将字符串插入到调试表中。

如果我跑

call UrlToId('http://mail4.z.uk/foo/bar');

我收到这些调试消息

UrlAuthority authority_tld=z.uk authority_id=1
UrlFqdn fqdn=mail4.z.uk fqdn_id=NULL
Url fqdn_id=NULL pathtext=/foo/bar

显然，问题是，fqdn_id=NULL但是，如果我手动重复执行SELECT id INFO fqdn_id并替换跟踪中指示的变量值的查询：

    SELECT id
    -- INTO fqdn_id
    FROM UrlFqdn
    WHERE authority=1 /* authority_id */ AND name='mail4.z.uk' /* fqdn */;

这1将按预期进行选择。

在select得到数据，为什么是变数NULL后来呢？

Answer 1

有一段时间，这个问题让我望而却步。我不喜欢SELECT ... INTO，所以我真的想把你的麻烦归咎于它，但没有。

问题来了。

SELECT id
  INTO fqdn_id
  FROM UrlFqdn
 WHERE authority=authority_id AND name=fqdn;

MySQL如何解释这个？

SELECT id
  INTO fqdn_id
  FROM UrlFqdn
 WHERE 'mail4' = 1 AND name = 'mail4.z.uk';
/* Impossible WHERE */
/* see DECLARE authority TEXT; */

“局部变量不应与表列同名。如果 SQL 语句（例如SELECT ... INTO语句）包含对列的引用和具有相同名称的声明的局部变量，则 MySQL 当前将引用解释为变量的名称。”

—  http://dev.mysql.com/doc/refman/5.7/en/local-variable-scope.html

对变量和列名使用相同的标识符会使调试变得一团糟，不管怎样。