使用聯接表中的if條件確定列數據

Question

我有一個運行良好的查詢，在這里我想尋求幫助的是如何合並條件以確定在給定列上顯示的值。 也就是說，如果表的某個列具有“ COA授權”的值，則添加的列日期應顯示日期，否則該列應顯示null

到目前為止，這是我的查詢

SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, CONCAT(u.fname,' ',u.lname) 'analyst' , t.date_added (//should show the value of date_added else NULL based on a condition for this column),
    FROM request r
    LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
    LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
    LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
    WHERE r.client_id='$cid'
    AND r.created_at BETWEEN '$start' AND '$end'
    AND a_s.department_id = '$dept'
    GROUP BY r.request_id
    ORDER BY `r`.`created_at` DESC "

Answer 1

我認為，在這種情況下，簡單的CASE語句就足夠了。

SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, 
CONCAT(u.fname,' ',u.lname) 'analyst', 
CASE WHEN ConditionColumn = 'Authorization of COA' THEN t.date_added 
ELSE NULL END AS 'date_added' 
    FROM request r
    LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
    LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
    LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
    WHERE r.client_id='$cid'

Answer 2

試試看：

SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, CONCAT(u.fname,' ',u.lname) 'analyst' , 
IFF(nameOfColumnToTest = 'Authorization of COA', t.date_added, null) AS DateAdded 
    FROM request r
    LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
    LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
    LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
    WHERE r.client_id='$cid'
    AND r.created_at BETWEEN '$start' AND '$end'
    AND a_s.department_id = '$dept'
    GROUP BY r.request_id
    ORDER BY `r`.`created_at` DESC "

Answer 3

在mysql和ms sql中，都可以使用case表達式（ mysql case ； ms sql case ）來有條件地將值分配給字段：

SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, CONCAT(u.fname,' ',u.lname) 'analyst' , 
    CASE WHEN Column_To_Test = 'Authorization of COA' THEN t.date_added
        ELSE null
    END AS date_added 
    FROM request r
    LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
    LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
    LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
    WHERE r.client_id='$cid'
    AND r.created_at BETWEEN '$start' AND '$end'
    AND a_s.department_id = '$dept'
    GROUP BY r.request_id
    ORDER BY `r`.`created_at` DESC "

這樣，解決方案在數據庫之間更加可移植。 Mysql提供了if（）函數和ms sql iif（）來實現上述目的，但是這些是特定於產品的解決方案。

Answer 4

嘗試使用Case語句之類的。

CASE WHEN （ Condition_1 AND Condition_2 OR ... Condition_N ） THEN （Value_1） ELSE （Value_1） END

這里的條件 _1可以是列記錄值，並且其列記錄值和值 _1的組合可以是單個值[Ex：Null或“ some string”]或嵌套sql語句返回的值，例如[Ex：（select count（*）from ...）]