mysql - 如何按第一个字母计算行数？

Question

这里有一个菜鸟问题！

我写了这个查询，但是“分组依据”非常愚蠢......那么，我该如何纠正这个问题？

SELECT
    COUNT(*) AS total,
    'x' as test
    FROM
contents
    WHERE name LIKE 'C%'
GROUP BY
    test
ORDER BY id ASC

欢迎使用不同的解决方案和有关表演的信息（也许使用 DISTINCT？）

提前致谢！

Answer 1

这应该与任何其他选项一样好 -

SELECT
    LEFT(name, 1) AS first_letter,
    COUNT(*) AS total
FROM contents
GROUP BY first_letter

如果您想一次针对单个字母运行此查询，您可以添加 WHERE 子句并删除 GROUP BY -

SELECT COUNT(*) AS total
FROM contents
WHERE name LIKE 'a%'

Answer 2

让我们剖析您的查询：

SELECT
    COUNT(*) AS total,
    'x' as test     <-- Why?
    FROM            <-- Bad formatting.
contents
    WHERE name LIKE 'C%'
GROUP BY
    test            <-- Removing 'x' and the whole GROUP BY has the same effect.
ORDER BY id ASC     <-- The result only contains one row - nothing to sort.

因此，返回一行和一个字段的查询，包含name以“C”开头的行数，如下所示：

SELECT COUNT(*)
FROM contents
WHERE name LIKE 'C%'

拥有领先优势的索引name将确保良好的性能。要了解原因，请查看SQL 索引剖析。

Answer 3

应该给你一切，以备不时之需

SELECT
  COUNT(*) AS total,
  test
FROM
 (SELECT substring(name,1,1) as test
 from contents) t
GROUP BY test