来自MySQL中多个表的COUNT(*)

Question

如何从MySQL中的多个表中选择COUNT(*)？

如:

SELECT COUNT(*) AS table1Count FROM table1 WHERE someCondition
JOIN?? 
SELECT COUNT(*) AS table2Count FROM table2 WHERE someCondition
CROSS JOIN? subqueries?
SELECT COUNT(*) AS table3Count FROM table3 WHERE someCondition

编辑:

目标是返回:

+-------------+-------------+-------------+
| table1Count | table2Count | table3Count |
+-------------+-------------+-------------+
| 14          | 27          | 0           |
+-------------+-------------+-------------+

Answer 1

您可以使用子查询来执行此操作,每个tableCount都有一个子查询:

SELECT
  (SELECT COUNT(*) FROM table1 WHERE someCondition) as table1Count, 
  (SELECT COUNT(*) FROM table2 WHERE someCondition) as table2Count,
  (SELECT COUNT(*) FROM table3 WHERE someCondition) as table3Count

Answer 2

您可以使用子查询执行此操作,例如:

select (SELECT COUNT(*) FROM table1 WHERE someCondition) as table1Count, 
       (SELECT COUNT(*) FROM table2 WHERE someCondition) as table2Count 

Answer 3

如果特定表没有条件，这里是从多个表中纯粹获取行数的简单方法。

笔记：

对于 InnoDB，这个计数是一个近似值。但是，对于 MyISAM，计数是准确的。

引用自文档：

行数。一些存储引擎，例如 MyISAM，存储确切的计数。对于其他存储引擎，比如 InnoDB，这个值是一个近似值，可能与实际值相差 40% 到 50%。在这种情况下，请使用 SELECT COUNT(*) 获得准确的计数。

使用该information_schema.tables表，您可以使用：

SELECT 
    table_name, 
    table_rows
FROM 
    information_schema.tables
WHERE
    table_name like 'my_table%';

输出：

table_name    table_rows
my_table_1    0
my_table_2    15
my_table_3    30

Answer 4

你可以使用UNION

  SELECT COUNT(*) FROM table1 WHERE someCondition
  UNION
  SELECT COUNT(*) FROM table2 WHERE someCondition
  UNION
  SELECT COUNT(*) FROM table3 WHERE someCondition