Question

这是一个菜鸟问题！

我写了这个查询，但是“group by”非常愚蠢...... 那么，我该如何纠正呢？

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

欢迎不同的解决方案和有关表演的信息（可能使用DISTINCT？）

提前感谢！

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

如果要一次为单个字母运行此查询，可以添加WHERE子句并删除GROUP BY -

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

Answer 2

让我们剖析你的问题：

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，返回包含一个字段的行的查询包含name以“C”开头的行数，如下所示：

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

拥有前沿为name的索引可确保良好的性能。要了解原因，请查看Anatomy of an SQL Index。

Answer 3

如果你想要它，

应该给你一切

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

如何按首字母计算行数？

3 个答案: