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

修改

目标是归还：

+-------------+-------------+-------------+
| 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

您可以使用UNION

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

Answer 4

如果在特定表上没有没有条件，那么这是一种从多个表中纯粹获取行数的简单方法。

注意：

对于InnoDB，此计数为近似值。但是，对于MyISAM，该计数是准确的。

引自docs：

行数。一些存储引擎（例如MyISAM）存储精确计数。对于其他存储引擎，例如InnoDB，此值为一个近似值，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

我之所以发布此答案，是因为我在该帖子中搜索了与帖子标题匹配的此方法。希望将来能对某人有所帮助。

Answer 5

尝试更改为：

SELECT 
    COUNT(table1.*) as t1,
    COUNT(table2.*) as t2,
    COUNT(table3.*) as t3 
FROM table1 
    LEFT JOIN tabel2 ON condition
    LEFT JOIN tabel3 ON condition

Answer 6

你可以这样做。

SELECT (select count(*) from table1) + (select count(*) from table2) as total_rows

您可以添加任意数量的表。

来自MySQL中多个表的COUNT（*）

6 个答案:

注意：

输出：