Question

我的SQL-Query有点问题。在我的数据库中，有3个表table1，table2，table3和一些字段。在这种情况下，每个表最多只有3个字段（id，field1，field2）是相关的，我试着在下面的查询中保持简单。

我有以下查询：

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

出于某种原因，我总是得到错误的结果，所以我决定添加

SELECT t2.id

和

AND t2.id = 1

测试它

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

现在在我的结果集中，仍然有t2.id！= 1的条目。我如何防止这种情况？

Answer 1

导致混淆的是OR。把它放在括号内：

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

使用现代明确的JOIN语法重写：

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

PostgreSQL - 使用特定查询获取错误的结果

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