PostgreSQL - 使用特定查詢得到錯誤的結果

Question

我的SQL查詢有一點問題。在我的數據庫中，有3個表格table1，table2，table3和一些字段。在這種情況下，每個表只有最多3個字段（id，field1，field2）是相關的，我試圖在下面的查詢中保持簡單。

我有以下查詢：

SELECT t1.field1, t1.field2 
FROM table1 t1, table2 t2, table3 t3 
WHERE t3.field1 = 'param1' 
    AND t3.id = t1.t3_id 
    AND t1.field2 = 'param2' 
    OR t1.field2 IS NULL 
    AND t2.field1 = 'param3' 
    AND t2.id = t1.t2_id 

出於某種原因，我總是得到錯誤的結果，所以我決定加入

SELECT t2.id

和

AND t2.id = 1

測試它

SELECT t1.field1, t1.field2, t2.id 
FROM table1 t1, table2 t2, table3 t3 
WHERE t3.field1 = 'param1' 
    AND t3.id = t1.t3_id 
    AND t1.field2 = 'param2' 
    OR t1.field2 IS NULL 
    AND t2.field1 = 'param3' 
    AND t2.id = t1.t2_id 
    AND t2.id = 1 

在我的結果集

現在，仍然有條目與t2.id！= 1，我怎樣才能避免這種情況？

Answer 1

這是導致混淆的OR。把它放在括號內：

SELECT t1.field1, t1.field2, t2.id 
FROM table1 t1, table2 t2, table3 t3 
WHERE t3.field1 = 'param1' 
    AND t3.id = t1.t3_id 
    AND (t1.field2 = 'param2' OR t1.field2 IS NULL) 
    AND t2.field1 = 'param3' 
    AND t2.id = t1.t2_id 
    AND t2.id = 1 

重新寫了現代化的，明確的JOIN語法：

SELECT t1.field1, t1.field2, t2.id 
FROM table1 t1 
JOIN table2 t2 ON t2.id = t1.t2_id 
JOIN table3 t3 ON t3.id = t1.t3_id 
WHERE t3.field1 = 'param1' 
    AND (t1.field2 = 'param2' OR t1.field2 IS NULL) 
    AND t2.field1 = 'param3' 
    AND t2.id = 1