mysql - SELECT … WHERE 没有 FROM 抛出语法错误

Question

我使用的是 MySQL 5.5.38，我试图用 SELECT 生成一个包含零行的结果。

使用不存在的 DUAL 表，它可以工作:

SELECT 123 FROM DUAL WHERE FALSE

但是普通版给出了语法错误:

SELECT 123 WHERE FALSE

1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE FALSE' at line 1

这是 MySQL 错误还是我遗漏了什么？

http://sqlfiddle.com/#!9/d41d8/1028

Answer 1

我找不到 documented 的语法，所以它在技术上不受支持:

SELECT
    [ALL | DISTINCT | DISTINCTROW ]
      [HIGH_PRIORITY]
      [STRAIGHT_JOIN]
      [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
      [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr [, select_expr ...]
    [FROM table_references
      [PARTITION partition_list]
    [WHERE where_condition]
    [GROUP BY {col_name | expr | position}
      [ASC | DESC], ... [WITH ROLLUP]]
    [HAVING where_condition]
    [ORDER BY {col_name | expr | position}
      [ASC | DESC], ...]
    [LIMIT {[offset,] row_count | row_count OFFSET offset}]
    [PROCEDURE procedure_name(argument_list)]
    [INTO OUTFILE 'file_name'
        [CHARACTER SET charset_name]
        export_options
      | INTO DUMPFILE 'file_name'
      | INTO var_name [, var_name]]
    [FOR UPDATE | LOCK IN SHARE MODE]]

如您所见，FROM 是可选的，但 WHERE 需要 FROM:

[FROM table_references
      [PARTITION partition_list]
    [WHERE where_condition]

无论如何，打印一行并立即将其过滤掉似乎不是特别有用，不是吗？

It might be useful for unioning multiple similar selects

这是一个不同的查询，因此是一个不同的语法...使用 FROM ;-)

SELECT *
FROM (
    SELECT 123 AS foo
    UNION
    SELECT 456
    UNION
    SELECT 789
) data
WHERE foo<>456

如果您只是在末尾附加一个普通的 WHERE，您只会过滤最后一个 SELECT。