Question

我需要能够检测单个查询中是否存在数据库和/或表，以便相应地执行操作。我有这个讨厌的查询工作：

SELECT * FROM

(SELECT COUNT(*) AS `database`
FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMATA.SCHEMA_NAME="database_name") AS foo,

(SELECT COUNT(*) AS `table`
FROM INFORMATION_SCHEMA.TABLES
WHERE table_schema = "database_name"
AND table_name = "table_name") AS bar

此查询返回：

database    table
1           0

但是......也许有更好的方法。

Answer 1

使用LEFT JOIN：

SELECT schemata.schema_name AS `database_name`, tables.table_name
FROM INFORMATION_SCHEMA.SCHEMATA schemata
LEFT JOIN INFORMATION_SCHEMA.TABLES tables 
  ON schemata.schema_name = tables.table_schema
  AND tables.table_name = "table_name"
WHERE SCHEMATA.SCHEMA_NAME="database_name"

Answer 2

这也行，但我不确定它是否更漂亮：）

SELECT
  MAX(CASE
      WHEN table_schema = 'database_name' THEN 1
      ELSE 0
      END) AS `database`
, MAX(CASE
      WHEN table_schema = 'database_name' AND table_name = 'table' THEN 1
      ELSE 0
      END) AS `table`
FROM information_schema.tables;

检测同一查询中是否存在数据库和表

2 个答案: