Question

我的这个mysql查询对每个选择都有限制，但我也想在整个查询中限制为1。我也试图围绕整个查询构建一个Select，但它仍然遍历所有表，即使第一个表中有结果，所以持续时间是相同的。我希望查询在得到结果后立即停止。对此有什么解决方案吗？

(SELECT count(*) FROM table1 i JOIN table1item it where it.columnId = 2 LIMIT 1)union
(SELECT count(*) FROM table2 i JOIN table2item it where it.columnId = 2 LIMIT 1)union
(SELECT count(*) FROM table3 i JOIN table3item it where it.columnId = 2 LIMIT 1)union
(SELECT count(*) FROM table4 i JOIN table4item it where it.columnId = 2 LIMIT 1)union
(SELECT count(*) FROM table5 i JOIN table5item it where it.columnId = 2 LIMIT 1)union
(SELECT count(*) FROM table6 i JOIN table6item it where it.columnId = 2 LIMIT 1)union
(SELECT count(*) FROM table7 i JOIN table7item it where it.columnId = 2 LIMIT 1)

Answer 1

添加UNION ALL以确保如果它们相同，您将从所有表中获得结果。

(SELECT count(*) FROM table1 i JOIN table1item it where it.columnId = 2) UNION ALL
(SELECT count(*) FROM table2 i JOIN table2item it where it.columnId = 2) UNION ALL
(SELECT count(*) FROM table3 i JOIN table3item it where it.columnId = 2) UNION ALL
(SELECT count(*) FROM table4 i JOIN table4item it where it.columnId = 2) UNION ALL
(SELECT count(*) FROM table5 i JOIN table5item it where it.columnId = 2) UNION ALL
(SELECT count(*) FROM table6 i JOIN table6item it where it.columnId = 2) UNION ALL
(SELECT count(*) FROM table7 i JOIN table7item it where it.columnId = 2)
LIMIT 1

Answer 2

一位同事发现了一个真正缩短查询时间的解决方案（在我的测试用例中为0.25而不是0.75）：

SELECT 
                case 
                               when count(*) > 0 
                                               then 1                                                  
                else (
    SELECT 
                case 
                               when count(*) > 0 
                                               then 1                                                  
                else (
    SELECT 
                case 
                               when count(*) > 0 
                                               then 1                                                  
                else (
    SELECT 
                case 
                               when count(*) > 0 
                                               then 1                                                  
                else (
    SELECT 
                case 
                               when count(*) > 0 
                                               then 1                                                  
                else (
    SELECT 
                case 
                               when count(*) > 0 
                                               then 1                                                  
                else (
    SELECT 
                case 
                               when count(*) > 0 
                                               then 1                                                  
                else 0
end as result
FROM table1 i JOIN table1item it where it.fooId = 1 LIMIT 1 
    )
end as result
FROM table2 i JOIN table2item it where it.fooId = 1 LIMIT 1 
    )
end as result
FROM table3 i JOIN table3item it where it.fooId = 1 LIMIT 1 
    )
end as result
FROM table4 i JOIN table4item it where it.fooId = 1 LIMIT 1 
    )
end as result
FROM table5 i JOIN table5item it where it.fooId = 1 LIMIT 1 
    )
end as result
FROM table6 i JOIN table6item it where it.fooId = 1 LIMIT 1 
    )
end as result
FROM table7 i JOIN table7item it where it.fooId = 1 LIMIT 1

限制多表联合

2 个答案: