Wie erzwinge ich die Verbindung in einem linken Join-Zweig?

Question

Ich muss links Join zu ganzen t2 + t3-Zweig, aber wenn ich einen passenden Join zwischen t1 und t2 finden kann, möchte ich die t2 und t3 Join erzwingen.

SELECT T1.name,T2.bob,T3.a 
    FROM T1 
LEFT JOIN T2 ON t1.id = t2.t1_id 
    JOIN T3 ON t2.id = T3.t2_id 

Wie lautet die Syntax?

Beispieldaten:

T1 [id,name] 
1 aaa 
2 bbb 
3 ccc 

T2 [id,t1_id,bob] 
1,1,777 
2,1,888 
2,2,999 

T3[id,t2_id,a] 
1,2,'yeh' 

Erwartetes Ergebnis:

[name] , [a] , [bob] 
    aaa , 'yeh' , 888 
    bbb , NULL , NULL 
    ccc , NULL , NULL 

Answer 1

EDIT: Diese Abfrage liefert das Ergebnis als von Ihren Beispieldaten erwartet - (Beachten Sie auch, wird diese Abfrage gemacht mit Hilfe von Treefrogs Antwort unten) -

SELECT t1.[name], t3.a, t2.bob 
FROM T2 as t2 
JOIN T3 as t3 ON t3.t2_id = t2.id 
RIGHT JOIN T1 as t1 ON t1.id = t2.t1_id 

Meine ältere Antwort -

SELECT a 
FROM T1 as t1 
INNER JOIN T2 as t2 ON t1.id = t2.t1_id 
LEFT JOIN T3 as t3 ON t2.id = t3.t2_id 

Answer 2

SELECT T1.a 
FROM T2 
JOIN T3 ON T3.t2_id = T2.id 
RIGHT JOIN T1 ON T1.id = T2.t1_id 

Answer 3

Beide werden Sie Ihr Ergebnis. Nicht sicher, was besser wäre.

VERSCHACHTELTE STATEMENT:

SELECT [name], NULL AS [a], NULL AS [bob] 
FROM t1 
LEFT OUTER JOIN t2 ON t1.id = t2.t1_id 
LEFT OUTER JOIN t3 ON t2.id = t3.t2_id 
WHERE t3.t2_id IS NULL 
AND (SELECT COUNT(*) FROM t1 AS t1b LEFT OUTER JOIN t2 AS t2b ON t1b.id = t2b.t1_id LEFT OUTER JOIN t3 AS t3b ON t2b.id = t3b.t2_id WHERE t1.id = t1b.id AND t3b.a IS NOT NULL) = 0 
UNION 
SELECT [name], [a], [bob] 
FROM t1 
LEFT OUTER JOIN t2 ON t1.id = t2.t1_id 
LEFT OUTER JOIN t3 ON t2.id = t3.t2_id 
WHERE t3.t2_id IS NOT NULL 
ORDER BY t1.name 

TEMPORARY TABLE:

CREATE TABLE #tmp_Rslt([name] varchar(50), [a] varchar(50), [bob] varchar(50)) 

--select matches 
INSERT INTO #tmp_Rslt 
SELECT [name], [a], [bob] 
FROM t1 
LEFT OUTER JOIN t2 ON t1.id = t2.t1_id 
LEFT OUTER JOIN t3 ON t2.id = t3.t2_id 
WHERE t3.t2_id IS NOT NULL 
ORDER BY t1.name 

--select t1's that didn't have matches 
INSERT INTO #tmp_Rslt 
SELECT [name], NULL AS [a], NULL AS [bob] 
FROM t1 
LEFT OUTER JOIN t2 ON t1.id = t2.t1_id 
LEFT OUTER JOIN t3 ON t2.id = t3.t2_id 
WHERE t3.t2_id IS NULL 
AND t1.[name] NOT IN (SELECT DISTINCT [name] FROM #tmp_Rslt) 

SELECT * 
FROM #tmp_Rslt 

--cleanup. 
DROP TABLE #tmp_Rslt 

Answer 4

Dies sollte in MySQL arbeiten

SELECT * FROM T1 
LEFT JOIN (T2 
INNER JOIN T3 ON T2.id=T3.t2_id 
) ON T1.id= T2.t1_id