INSERT和WHERE不存在的怪異SQL行爲

Question

我有這個查詢，如果表中沒有相同ID的金額等於或高於此值，應該插入一個金額。

我嘗試這樣做的MySQL 5.1和MSSQL 2K5下：

的MySQL

INSERT IGNORE INTO測試（ID，金額） SELECT 6，50 FROM測試 WHERE NOT EXISTS（SELECT 1 FROM測試WHERE量> = 50和ID = 6）LIMIT 1

MSSQL

INSERT INTO噸EST（ID，金額） SELECT 6，50 FROM測試 WHERE NOT EXISTS（SELECT TOP 11 FROM測試WHERE量> = 50和ID = 6）

該查詢工作正常，如果已經存在至少一個條目在桌子裏。如果表格是空的，它將無法工作。這與MySQL（5.1）和MSSQL（2005）下的行爲是一樣的。 我不明白爲什麼。任何人都有解釋和方法來解決這個查詢工作，即使表是完全空的？

編輯：我需要這個MySQL的主要...

更新：我開始了新的問題，特別爲MySQL： MySQL Problem with inserting a row with certain conditions

Answer 1

它失敗，因爲select語句根據返回的行數選擇6和50的值。由於你的表是空的 - 沒有行返回這些值。

其修改爲

INSERT INTO test (id, amount) SELECT 6, 50 WHERE NOT EXISTS (SELECT TOP 1 1 FROM test WHERE amount >= 50 AND id = 6) 

Answer 2

更改您的查詢，如下所示：

INSERT INTO test (id, amount) 
SELECT 6, 50 FROM test 
WHERE (SELECT count(*) FROM test WHERE amount >= 50 AND id = 6) = 0 

Answer 3

我想一步一步來：

-- create a test-table with one entry 
SELECT id = 1, amount = 50 
into #TEST 

-- see if statement works when table is not empty 
INSERT INTO #test (id, amount) 
SELECT id = 6, amount = 50 
FROM #test 
WHERE NOT EXISTS (SELECT * FROM #test WHERE amount >= 50 AND id = 6) 

-- show content of test-table 
select * from #test 

-- empty the test table 
truncate table #test 

-- try to insert a first dataentry 
INSERT INTO #test (id, amount) 
SELECT id = 6, amount = 50 
FROM #test 
WHERE NOT EXISTS (SELECT * FROM #test WHERE amount >= 50 AND id = 6) 
union 
SELECT id = 6, amount = 50 
where (SELECT C = COUNT(*) FROM #test WHERE amount >= 50 AND id = 6) = 0 

-- show content of test-table 
select * from #test 

-- remove Test-table 
drop table #test 

首先，我試圖重現行爲，是很奇怪的。工會 - 認爲管理空表的案件，並認爲這工作正常。

Answer 4

好的，這是真正的問題。試試這個，使用第二個表（＃測試2）：

-- create a test-table with one entry 
SELECT id = 1, amount = 50 
into #TEST 

-- create a second test-table with one entry 
SELECT id = 1, amount = 50 
into #TEST2 

-- see if statement works when table is not empty 
INSERT INTO #test (id, amount) 
SELECT id = 6, amount = 50 
FROM #test 
WHERE NOT EXISTS (SELECT * FROM #test WHERE amount >= 50 AND id = 6) 

-- show content of test-table 
select * from #test 

-- empty the test table 
truncate table #test 

INSERT INTO #test (id, amount) SELECT 6, 50 WHERE NOT EXISTS (SELECT TOP 1 1 FROM #test WHERE amount >= 50 AND id = 6) 

-- try to insert a first dataentry 
INSERT INTO #test (id, amount) 
SELECT id = 6, amount = 50 
FROM #test 
WHERE NOT EXISTS (SELECT * FROM #test WHERE amount >= 50 AND id = 6) 
union 
SELECT id = 6, amount = 50 
from #test2 
where (SELECT C = COUNT(*) FROM #test) = 0 


-- show content of test-table 
select * from #test 

-- remove Test-table 
drop table #test 

（假設你知道與前述＃表，這是在MS-SQL的臨時表）。

BonyT已經說明了原因。