如何加入并非所有行都可用的行？
How to join rows where not all is available?
在 SQL 中可能会丢失一两行时，我如何加入一些有序的（按 StNr）行？我有以下 table。我们称它为测量值：
id DtTm Cd MeasNr StNr Var1 Var2 Var3
-----------------------------------------------------
1 2021-10-25 abc 1 2 1.5 3.7 4.6
2 2021-10-25 abc 1 3 NULL 2.6 2.8
3 2021-10-25 def 3 1 3.5 NULL 3.6
4 2021-10-25 abc 2 1 2.6 NULL 1.5
5 2021-10-25 def 3 3 NULL 2.8 3.1
6 2021-10-25 abc 2 2 3.6 2.5 2.7
7 2021-10-25 xyz 1 2 2.2 3.0 2.6
8 2021-10-25 xyz 1 3 1.8 2.0 1.9
9 2021-10-25 xyz 1 1 3.6 1.5 3.1
我想加入代码 (Cd) 和测量编号 (MeasNr) 相同的行，并在变量 (Var1、Var2、Var3) 前加上它们的站号 (StNr)。在此示例中，3 行彼此属于（StNr：1、2、3）。
期望的输出：
Cd MeasNr st1_Var1 st1_Var2 st1_Var3 st2_Var1 st2_Var2 st2_Var3 st3_Var1 st3_Var2 st3_Var3
-----------------------------------------------------------------------------------------------------
abc 1 NULL NULL NULL 1.5 3.7 4.6 NULL 2.6 2.8
abc 2 2.6 NULL 1.5 3.6 2.5 2.7 NULL NULL NULL
def 3 3.5 NULL 3.6 NULL NULL NULL NULL 2.8 3.1
xyz 1 3.6 1.5 3.1 2.2 3.0 2.6 1.8 2.0 1.9
我尝试了以下 JOIN：
SELECT st1.Cd, st1.MeasNr,
st1.Var1 as st1_Var1, st1.Var2 as st1_Var2, st1.Var3 as st1_Var3,
st2.Var1 as st2_Var1, st2.Var2 as st2_Var2, st2.Var3 as st2_Var3,
st3.Var1 as st3_Var1, st3.Var2 as st3_Var2, st3.Var3 as st3_Var3
FROM Measurements as st1
LEFT JOIN Measurements as st2 on (st1.Cd = st2.Cd and st1.MeasNr = st2.MeasNr) and (st2.StNr = 2)
LEFT JOIN Measurements as st3 on (st1.Cd = st3.Cd and st1.MeasNr = st3.MeasNr) and (st3.StNr = 3)
where st1.StNr = 1
然而，“abc”的第一个测量值未列出（没有 StNr=1 的行，因此没有可加入的内容）。我怎样才能获得第一条记录呢？我也尝试了 FULL JOIN，但没有成功。
这是我的 SELECT 完全连接：
SELECT
CASE
WHEN st1.DtTm IS NOT NULL THEN st1.DtTm
WHEN st2.DtTm IS NOT NULL THEN st2.DtTm
WHEN st3.DtTm IS NOT NULL THEN st3.DtTm
END AS DtTime,
CASE
WHEN st1.Cd IS NOT NULL THEN st1.Cd
WHEN st2.Cd IS NOT NULL THEN st2.Cd
WHEN st3.Cd IS NOT NULL THEN st3.Cd
END AS Cd,
CASE
WHEN st1.MeasNr IS NOT NULL THEN st1.MeasNr
WHEN st2.MeasNr IS NOT NULL THEN st2.MeasNr
WHEN st3.MeasNr IS NOT NULL THEN st3.MeasNr
END AS MeasNr,
st1.Var1 AS st1_Var1,
st1.Var2 AS st1_Var2,
st1.Var3 AS st1_Var3,
st2.Var1 AS st2_Var1,
st2.Var2 AS st2_Var2,
st2.Var3 AS st2_Var3,
st3.Var1 AS st3_Var1,
st3.Var2 AS st3_Var2,
st3.Var3 AS st3_Var3
FROM (SELECT
DtTm, Cd, MeasNr, Var1, Var2, Var3
FROM Measurements
WHERE StNr = 1) AS st1
FULL JOIN (SELECT
DtTm, Cd, MeasNr, Var1, Var2, Var3
FROM Measurements
WHERE StNr = 2) AS st2 ON (st1.Cd = st2.Cd AND st1.MeasNr = st2.MeasNr)
FULL JOIN (SELECT
DtTm, Cd, MeasNr, Var1, Var2, Var3
FROM Measurements
WHERE StNr = 3) AS st3 ON (st1.Cd = st3.Cd AND st1.MeasNr = st3.MeasNr)
通过这种方法，我有缺失的数据，但不是在一行中。
也许这不是一个很好的方法，但它确实有效：
with MyTable (Cd, MeasNr, st1_Var1, st1_Var2, st1_Var3, st2_Var1, st2_Var2, st2_Var3, st3_Var1, st3_Var2, st3_Var3)
as
(
select Cd, MeasNr, Var1 as st1_Var1, Var2 as st1_Var2, Var3 as st1_Var3, null as st2_Var1, null as st2_Var2, null as st2_Var3, null as st3_Var1, null as st3_Var2, null as st3_Var3 from Measurements st1 where StNr = 1
union
select Cd, MeasNr, null, null, null, Var1 as st2_Var1, Var2 as st2_Var2, Var3 as st2_Var3, null, null, null from Measurements st1 where StNr = 2
union
select Cd, MeasNr, null, null, null, null, null, null, Var1 as st3_Var1, Var2 as st3_Var2, Var3 as st3_Var3 from Measurements st1 where StNr = 3
)
select Cd, MeasNr, sum(st1_Var1) st1_Var1, sum(st1_Var2) st1_Var2, sum(st1_Var3) st1_Var3,
sum(st2_Var1) st2_Var1, sum(st2_Var2) st2_Var2, sum(st2_Var3) st2_Var3,
sum(st3_Var1) st3_Var1, sum(st3_Var2) st3_Var2, sum(st3_Var3) st3_Var3
from MyTable
group by Cd, MeasNr
order by Cd, MeasNr
我在这里粘贴了带有变量 table 的代码，也许它对测试有用：
declare @Measurements as table (
id int,
DtTm datetime,
Cd char(3),
MeasNr int,
StNr int,
Var1 decimal(5,1),
Var2 decimal(5,1),
Var3 decimal(5,1)
)
insert into @Measurements values (1, '25/10/21', 'abc', 1, 2, 1.5, 3.7, 4.6)
insert into @Measurements values (2, '25/10/21', 'abc', 1, 3, null, 2.6, 2.8)
insert into @Measurements values (3, '25/10/21', 'def', 3, 1, 3.5, null, 3.6)
insert into @Measurements values (4, '25/10/21', 'abc', 2, 1, 2.6, null, 1.5)
insert into @Measurements values (5, '25/10/21', 'def', 3, 3, null, 2.8, 3.1)
insert into @Measurements values (6, '25/10/21', 'abc', 2, 2, 3.6, 2.5, 2.7)
insert into @Measurements values (7, '25/10/21', 'xyz', 1, 2, 2.2, 3.0, 2.6)
insert into @Measurements values (8, '25/10/21', 'xyz', 1, 3, 1.8, 2.0, 1.9)
insert into @Measurements values (9, '25/10/21', 'xyz', 1, 1, 3.6, 1.5, 3.1);
with MyTable (Cd, MeasNr, st1_Var1, st1_Var2, st1_Var3, st2_Var1, st2_Var2, st2_Var3, st3_Var1, st3_Var2, st3_Var3)
as
(
select Cd, MeasNr, Var1 as st1_Var1, Var2 as st1_Var2, Var3 as st1_Var3, null as st2_Var1, null as st2_Var2, null as st2_Var3, null as st3_Var1, null as st3_Var2, null as st3_Var3 from @Measurements st1 where StNr = 1
union
select Cd, MeasNr, null, null, null, Var1 as st2_Var1, Var2 as st2_Var2, Var3 as st2_Var3, null, null, null from @Measurements st1 where StNr = 2
union
select Cd, MeasNr, null, null, null, null, null, null, Var1 as st3_Var1, Var2 as st3_Var2, Var3 as st3_Var3 from @Measurements st1 where StNr = 3
)
select Cd, MeasNr, sum(st1_Var1) st1_Var1, sum(st1_Var2) st1_Var2, sum(st1_Var3) st1_Var3,
sum(st2_Var1) st2_Var1, sum(st2_Var2) st2_Var2, sum(st2_Var3) st2_Var3,
sum(st3_Var1) st3_Var1, sum(st3_Var2) st3_Var2, sum(st3_Var3) st3_Var3
from MyTable
group by Cd, MeasNr
order by Cd, MeasNr
对于 MySQL < v8 不支持常见的 table 表达式 WITH:
SELECT Cd, MeasNr,
SUM(c11) AS st1_Var1, SUM(c12) AS st1_Var2, SUM(c13) AS st1_Var3,
SUM(c21) AS st2_Var1, SUM(c22) AS st2_Var2, SUM(c23) AS st2_Var3,
SUM(c31) AS st3_Var1, SUM(c32) AS st3_Var2, SUM(c33) AS st3_Var3
FROM (
SELECT DISTINCT Cd, MeasNr,
Var1 as c11, Var2 as c12, Var3 as c13,
NULL as c21, NULL as c22, NULL as c23,
NULL as c31, NULL as c32, NULL as c33
FROM Measurements WHERE StNr = 1
UNION
SELECT DISTINCT Cd, MeasNr,
NULL as c11, NULL as c12, NULL as c13,
Var1 as c21, Var2 as c22, Var3 as c23,
NULL as c31, NULL as c32, NULL as c33
FROM Measurements WHERE StNr = 2
UNION
SELECT DISTINCT Cd, MeasNr,
NULL as c11, NULL as c12, NULL as c13,
NULL as c21, NULL as c22, NULL as c23,
Var1 as c31, Var2 as c32, Var3 as c33
FROM Measurements WHERE StNr = 3
) t1
GROUP BY Cd, MeasNr
ORDER BY Cd, MeasNr
这看起来像是一个简单的条件聚合案例，也称为旋转。
由于是多列数据透视表，直接使用MAX(CASE WHEN
会简单很多
注意：不需要连接
SELECT
m.Cd,
m.MeasNr,
MAX(CASE WHEN StNr = 1 THEN Var1 END) AS st1_Var1,
MAX(CASE WHEN StNr = 1 THEN Var2 END) AS st1_Var2,
MAX(CASE WHEN StNr = 1 THEN Var3 END) AS st1_Var3,
MAX(CASE WHEN StNr = 2 THEN Var1 END) AS st2_Var1,
MAX(CASE WHEN StNr = 2 THEN Var2 END) AS st2_Var2,
MAX(CASE WHEN StNr = 2 THEN Var3 END) AS st2_Var3,
MAX(CASE WHEN StNr = 3 THEN Var1 END) AS st3_Var1,
MAX(CASE WHEN StNr = 3 THEN Var2 END) AS st3_Var2,
MAX(CASE WHEN StNr = 3 THEN Var3 END) AS st4_Var3
FROM Measurements m
GROUP BY
m.Cd,
m.MeasNr
ORDER BY
m.Cd,
m.MeasNr;
在 SQL 中可能会丢失一两行时，我如何加入一些有序的（按 StNr）行？我有以下 table。我们称它为测量值：
id DtTm Cd MeasNr StNr Var1 Var2 Var3
-----------------------------------------------------
1 2021-10-25 abc 1 2 1.5 3.7 4.6
2 2021-10-25 abc 1 3 NULL 2.6 2.8
3 2021-10-25 def 3 1 3.5 NULL 3.6
4 2021-10-25 abc 2 1 2.6 NULL 1.5
5 2021-10-25 def 3 3 NULL 2.8 3.1
6 2021-10-25 abc 2 2 3.6 2.5 2.7
7 2021-10-25 xyz 1 2 2.2 3.0 2.6
8 2021-10-25 xyz 1 3 1.8 2.0 1.9
9 2021-10-25 xyz 1 1 3.6 1.5 3.1
我想加入代码 (Cd) 和测量编号 (MeasNr) 相同的行，并在变量 (Var1、Var2、Var3) 前加上它们的站号 (StNr)。在此示例中，3 行彼此属于（StNr：1、2、3）。
期望的输出：
Cd MeasNr st1_Var1 st1_Var2 st1_Var3 st2_Var1 st2_Var2 st2_Var3 st3_Var1 st3_Var2 st3_Var3
-----------------------------------------------------------------------------------------------------
abc 1 NULL NULL NULL 1.5 3.7 4.6 NULL 2.6 2.8
abc 2 2.6 NULL 1.5 3.6 2.5 2.7 NULL NULL NULL
def 3 3.5 NULL 3.6 NULL NULL NULL NULL 2.8 3.1
xyz 1 3.6 1.5 3.1 2.2 3.0 2.6 1.8 2.0 1.9
我尝试了以下 JOIN：
SELECT st1.Cd, st1.MeasNr,
st1.Var1 as st1_Var1, st1.Var2 as st1_Var2, st1.Var3 as st1_Var3,
st2.Var1 as st2_Var1, st2.Var2 as st2_Var2, st2.Var3 as st2_Var3,
st3.Var1 as st3_Var1, st3.Var2 as st3_Var2, st3.Var3 as st3_Var3
FROM Measurements as st1
LEFT JOIN Measurements as st2 on (st1.Cd = st2.Cd and st1.MeasNr = st2.MeasNr) and (st2.StNr = 2)
LEFT JOIN Measurements as st3 on (st1.Cd = st3.Cd and st1.MeasNr = st3.MeasNr) and (st3.StNr = 3)
where st1.StNr = 1
然而，“abc”的第一个测量值未列出（没有 StNr=1 的行，因此没有可加入的内容）。我怎样才能获得第一条记录呢？我也尝试了 FULL JOIN，但没有成功。
这是我的 SELECT 完全连接：
SELECT
CASE
WHEN st1.DtTm IS NOT NULL THEN st1.DtTm
WHEN st2.DtTm IS NOT NULL THEN st2.DtTm
WHEN st3.DtTm IS NOT NULL THEN st3.DtTm
END AS DtTime,
CASE
WHEN st1.Cd IS NOT NULL THEN st1.Cd
WHEN st2.Cd IS NOT NULL THEN st2.Cd
WHEN st3.Cd IS NOT NULL THEN st3.Cd
END AS Cd,
CASE
WHEN st1.MeasNr IS NOT NULL THEN st1.MeasNr
WHEN st2.MeasNr IS NOT NULL THEN st2.MeasNr
WHEN st3.MeasNr IS NOT NULL THEN st3.MeasNr
END AS MeasNr,
st1.Var1 AS st1_Var1,
st1.Var2 AS st1_Var2,
st1.Var3 AS st1_Var3,
st2.Var1 AS st2_Var1,
st2.Var2 AS st2_Var2,
st2.Var3 AS st2_Var3,
st3.Var1 AS st3_Var1,
st3.Var2 AS st3_Var2,
st3.Var3 AS st3_Var3
FROM (SELECT
DtTm, Cd, MeasNr, Var1, Var2, Var3
FROM Measurements
WHERE StNr = 1) AS st1
FULL JOIN (SELECT
DtTm, Cd, MeasNr, Var1, Var2, Var3
FROM Measurements
WHERE StNr = 2) AS st2 ON (st1.Cd = st2.Cd AND st1.MeasNr = st2.MeasNr)
FULL JOIN (SELECT
DtTm, Cd, MeasNr, Var1, Var2, Var3
FROM Measurements
WHERE StNr = 3) AS st3 ON (st1.Cd = st3.Cd AND st1.MeasNr = st3.MeasNr)
通过这种方法，我有缺失的数据，但不是在一行中。
也许这不是一个很好的方法，但它确实有效：
with MyTable (Cd, MeasNr, st1_Var1, st1_Var2, st1_Var3, st2_Var1, st2_Var2, st2_Var3, st3_Var1, st3_Var2, st3_Var3)
as
(
select Cd, MeasNr, Var1 as st1_Var1, Var2 as st1_Var2, Var3 as st1_Var3, null as st2_Var1, null as st2_Var2, null as st2_Var3, null as st3_Var1, null as st3_Var2, null as st3_Var3 from Measurements st1 where StNr = 1
union
select Cd, MeasNr, null, null, null, Var1 as st2_Var1, Var2 as st2_Var2, Var3 as st2_Var3, null, null, null from Measurements st1 where StNr = 2
union
select Cd, MeasNr, null, null, null, null, null, null, Var1 as st3_Var1, Var2 as st3_Var2, Var3 as st3_Var3 from Measurements st1 where StNr = 3
)
select Cd, MeasNr, sum(st1_Var1) st1_Var1, sum(st1_Var2) st1_Var2, sum(st1_Var3) st1_Var3,
sum(st2_Var1) st2_Var1, sum(st2_Var2) st2_Var2, sum(st2_Var3) st2_Var3,
sum(st3_Var1) st3_Var1, sum(st3_Var2) st3_Var2, sum(st3_Var3) st3_Var3
from MyTable
group by Cd, MeasNr
order by Cd, MeasNr
我在这里粘贴了带有变量 table 的代码，也许它对测试有用：
declare @Measurements as table (
id int,
DtTm datetime,
Cd char(3),
MeasNr int,
StNr int,
Var1 decimal(5,1),
Var2 decimal(5,1),
Var3 decimal(5,1)
)
insert into @Measurements values (1, '25/10/21', 'abc', 1, 2, 1.5, 3.7, 4.6)
insert into @Measurements values (2, '25/10/21', 'abc', 1, 3, null, 2.6, 2.8)
insert into @Measurements values (3, '25/10/21', 'def', 3, 1, 3.5, null, 3.6)
insert into @Measurements values (4, '25/10/21', 'abc', 2, 1, 2.6, null, 1.5)
insert into @Measurements values (5, '25/10/21', 'def', 3, 3, null, 2.8, 3.1)
insert into @Measurements values (6, '25/10/21', 'abc', 2, 2, 3.6, 2.5, 2.7)
insert into @Measurements values (7, '25/10/21', 'xyz', 1, 2, 2.2, 3.0, 2.6)
insert into @Measurements values (8, '25/10/21', 'xyz', 1, 3, 1.8, 2.0, 1.9)
insert into @Measurements values (9, '25/10/21', 'xyz', 1, 1, 3.6, 1.5, 3.1);
with MyTable (Cd, MeasNr, st1_Var1, st1_Var2, st1_Var3, st2_Var1, st2_Var2, st2_Var3, st3_Var1, st3_Var2, st3_Var3)
as
(
select Cd, MeasNr, Var1 as st1_Var1, Var2 as st1_Var2, Var3 as st1_Var3, null as st2_Var1, null as st2_Var2, null as st2_Var3, null as st3_Var1, null as st3_Var2, null as st3_Var3 from @Measurements st1 where StNr = 1
union
select Cd, MeasNr, null, null, null, Var1 as st2_Var1, Var2 as st2_Var2, Var3 as st2_Var3, null, null, null from @Measurements st1 where StNr = 2
union
select Cd, MeasNr, null, null, null, null, null, null, Var1 as st3_Var1, Var2 as st3_Var2, Var3 as st3_Var3 from @Measurements st1 where StNr = 3
)
select Cd, MeasNr, sum(st1_Var1) st1_Var1, sum(st1_Var2) st1_Var2, sum(st1_Var3) st1_Var3,
sum(st2_Var1) st2_Var1, sum(st2_Var2) st2_Var2, sum(st2_Var3) st2_Var3,
sum(st3_Var1) st3_Var1, sum(st3_Var2) st3_Var2, sum(st3_Var3) st3_Var3
from MyTable
group by Cd, MeasNr
order by Cd, MeasNr
对于 MySQL < v8 不支持常见的 table 表达式 WITH:
SELECT Cd, MeasNr,
SUM(c11) AS st1_Var1, SUM(c12) AS st1_Var2, SUM(c13) AS st1_Var3,
SUM(c21) AS st2_Var1, SUM(c22) AS st2_Var2, SUM(c23) AS st2_Var3,
SUM(c31) AS st3_Var1, SUM(c32) AS st3_Var2, SUM(c33) AS st3_Var3
FROM (
SELECT DISTINCT Cd, MeasNr,
Var1 as c11, Var2 as c12, Var3 as c13,
NULL as c21, NULL as c22, NULL as c23,
NULL as c31, NULL as c32, NULL as c33
FROM Measurements WHERE StNr = 1
UNION
SELECT DISTINCT Cd, MeasNr,
NULL as c11, NULL as c12, NULL as c13,
Var1 as c21, Var2 as c22, Var3 as c23,
NULL as c31, NULL as c32, NULL as c33
FROM Measurements WHERE StNr = 2
UNION
SELECT DISTINCT Cd, MeasNr,
NULL as c11, NULL as c12, NULL as c13,
NULL as c21, NULL as c22, NULL as c23,
Var1 as c31, Var2 as c32, Var3 as c33
FROM Measurements WHERE StNr = 3
) t1
GROUP BY Cd, MeasNr
ORDER BY Cd, MeasNr
这看起来像是一个简单的条件聚合案例，也称为旋转。
由于是多列数据透视表，直接使用MAX(CASE WHEN
注意：不需要连接
SELECT
m.Cd,
m.MeasNr,
MAX(CASE WHEN StNr = 1 THEN Var1 END) AS st1_Var1,
MAX(CASE WHEN StNr = 1 THEN Var2 END) AS st1_Var2,
MAX(CASE WHEN StNr = 1 THEN Var3 END) AS st1_Var3,
MAX(CASE WHEN StNr = 2 THEN Var1 END) AS st2_Var1,
MAX(CASE WHEN StNr = 2 THEN Var2 END) AS st2_Var2,
MAX(CASE WHEN StNr = 2 THEN Var3 END) AS st2_Var3,
MAX(CASE WHEN StNr = 3 THEN Var1 END) AS st3_Var1,
MAX(CASE WHEN StNr = 3 THEN Var2 END) AS st3_Var2,
MAX(CASE WHEN StNr = 3 THEN Var3 END) AS st4_Var3
FROM Measurements m
GROUP BY
m.Cd,
m.MeasNr
ORDER BY
m.Cd,
m.MeasNr;