MySQL

Question

我有一个正常工作的 MySQL:

SELECT
      tblprocedimento.idProcedimento,
      tblprocedimento.tituloProcedimento,
      tblprocedimento.tipoProximoS,
      tblprocedimento.numeroProximoS,
      tblprocedimento.tipoProximoN,
      tblprocedimento.numeroProximoN,
      ap1.nomeProximo AS nomeProximoS,
      ap2.nomeProximo AS nomeProximoN,
      tblauxtextoprocedimento.textoProcedimento,
      tblauxprocedimento.sTipificacao,
      tblNivel1.nivel1,
      tblNivel2.nivel2,
      tblNivel3.nivel3,
      tblNivel4.nivel4   
    FROM
    tblAuxTextoProcedimento
    LEFT JOIN 
    tblAuxProcedimento 
    ON 
    tblAuxTextoProcedimento.idTextoProcedimento = tblAuxProcedimento.sTextoProcedimento,
    tblNivel4 INNER JOIN (tblNivel3
    INNER JOIN (tblNivel2
    INNER JOIN (tblNivel1
    INNER JOIN tblAuxTipificacao
    ON tblNivel1.idNivel1 = tblAuxTipificacao.sNivel1)
    ON tblNivel2.idNivel2 = tblAuxTipificacao.sNivel2)
    ON tblNivel3.idNivel3 = tblAuxTipificacao.sNivel3)
    ON tblNivel4.idNivel4 = tblAuxTipificacao.sNivel4,
      tblprocedimento,
      tblauxproximo As ap1,
      tblauxproximo As ap2
    WHERE
      ap2.idProximo = tblprocedimento.sProxN AND
      ap1.idProximo = tblprocedimento.sProxS AND
      tblprocedimento.idProcedimento = 1 AND
      tblAuxProcedimento.sProcedimento = 1 AND
      tblAuxTipificacao.idTipificacao = 130

我想知道是否可以在此查询中仅查询 tblNivel1、tblNivel2、tblNivel3 和 tblNivel4 IF tblAuxTipificacao.idTipificacao <> value。

这样做的最佳做法是什么，或者我应该重写查询。

Answer 1

这些是条件联接。当您看到需要时，想想 Left Join。在以下示例中，type 和 proof 列显示概念有效。

这比很多人想的要好理解 case when ... 当问题被问到的时候。但这显然不是我们正在谈论的。

因此，重申一下，下面的证明列证明可以访问正确的 table。如果需要，您可以使用 case when 或 if 将其进一步包装在另一个嵌套的派生 table 中，并限制行输出。

稍后我可能有时间将其显示为编辑。

架构

create table items
(   sku varchar(20) primary key,
    type varchar(20) not null
);

insert items(sku,type) values ('0101','pencil'),('0292','pen'),('0294','pen');


create table pencils
(   sku varchar(20) primary key,
    leadType varchar(20) not null,
    color int not null,
    proof int not null
    -- FK not shown
);

insert pencils(sku,leadType,color,proof) values ('0101','No2',7,100);

create table pens
(   sku varchar(20) primary key,
    inkType varchar(20) not null,
    color int not null,
    erasable bool not null,
    proof int not null
    -- FK not shown
);

insert pens(sku,inkType,color,erasable,proof) values 
('0292','Squid',2,false,200),
('0294','Shrimp',33,true,300);

条件加入查询

select i.sku,i.type,p1.proof as PencilProof,p2.proof as PenProof  
FROM items i 
LEFT JOIN pencils p1 on p1.sku = i.sku and i.type = 'pencil' 
LEFT JOIN pens p2 on p2.sku = i.sku and i.type = 'pen';

+------+--------+-------------+----------+
| sku  | type   | PencilProof | PenProof |
+------+--------+-------------+----------+
| 0101 | pencil |         100 |     NULL |
| 0292 | pen    |        NULL |      200 |
| 0294 | pen    |        NULL |      300 |
+------+--------+-------------+----------+

派生的使用table

这使用外包装简化了输出，包括 CASE/WHEN，并使用上面的查询：

select sku,type, 
CASE type 
    WHEN 'pencil' THEN PencilProof 
    WHEN 'pen' THEN PenProof 
    ELSE -1 
END as Proof 
from 
(   select i.sku,i.type,p1.proof as PencilProof,p2.proof as PenProof 
    FROM items i 
    LEFT JOIN pencils p1 on p1.sku = i.sku and i.type = 'pencil' 
    LEFT JOIN pens p2 on p2.sku = i.sku and i.type = 'pen' 
) xDerived; -- every derived table needs a name (even if not used explicitly)

+------+--------+-------+
| sku  | type   | Proof |
+------+--------+-------+
| 0101 | pencil |   100 |
| 0292 | pen    |   200 |
| 0294 | pen    |   300 |
+------+--------+-------+

MySQL - 条件加入

MySQL - Conditional Join

conditional

架构

条件加入查询

派生的使用table