在带有 CASE 语句的 Sqlite 查询中处于 DRY

Question

在以下（有效）查询中：

SELECT q.id, q.section_id, q.type, q.required, q.condition,
CASE WHEN (t1.text_1 IS NULL) THEN
    CASE WHEN ((SELECT t2.text_1 FROM translations t2 WHERE t2.item_id = q.id AND t2.item_model = 'questions' AND t2.language = 'en' LIMIT 1) IS NULL) THEN 
        (SELECT t3.text_1 FROM translations t3 WHERE t3.item_id = q.id AND t3.item_model = 'questions' LIMIT 1)
    ELSE
        (SELECT t2.text_1 FROM translations t2 WHERE t2.item_id = q.id AND t2.item_model = 'questions' AND t2.language = 'en' LIMIT 1)
    END
ELSE
    t1.text_1
END
AS translation
FROM questions q
LEFT JOIN translations t1 ON t1.item_id = q.id
    AND t1.item_model = 'questions'
    AND t1.language = 'fr'
ORDER BY q.position

可以看到(SELECT t2.text_1 FROM translations t2 WHERE t2.item_id = q.id AND t2.item_model = 'questions' AND t2.language = 'en' LIMIT 1)部分重复了两次（第一次检查是否为null，第二次获取值）。

重复相同的查询会不会是性能问题（我猜是）？
是否有更好的方法来重写此查询，即 DRY？

Answer 1

您可以用 coalesce() 函数替换内部 CASE 语句：

coalesce(SELECT t2.text_1 FROM translations t2 WHERE t2.item_id = q.id AND t2.item_model = 'questions' AND t2.language = 'en' LIMIT 1,
SELECT t3.text_1 FROM translations t3 WHERE t3.item_id = q.id AND t3.item_model = 'questions' LIMIT 1)

来自documenation：

coalesce(X,Y,...)

The coalesce() function returns a copy of its first non-NULL argument, or NULL if all arguments are NULL. Coalesce() must have at least 2 arguments.

类似的是ifnull()函数。

在带有 CASE 语句的 Sqlite 查询中处于 DRY

Being DRY in a Sqlite query with CASE statement

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dry

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