**問題：**資料庫2張表做關聯，a表字段a，b表字段b，如果a列包含b列的內容，就關聯，這個應該怎麼寫？mysql資料庫
資料表示例如下：
一、
with cte1 as (
select 1 as fa,
'a' as fb,
'c' as fc
union all
select 2 as fa,
'b' as fb,
'd' as fc
union all
select 3 as fa,
'c' as fb,
'x' as fc
), cte2 as (
select 1 as fa,
'c' as fb
union all
select 2 as fa,
'no' as fb
union all
select 3 as fa,
'x' as fb
), cte21 as (
/*新增存在標準*/
select a.*,
(case when (select count(1
) from cte1 x where x.fc = a.fb)
>
0 then 1
else
0 end) as fexist
from cte2 a
)select x1.fa,
x1.fb,
x1.fc,
(case when x2.fexist =
1 then x2.fa
else null
end) as fa2,
(case when x2.fexist =
1 then x2.fb
else null
end) as fbc
from cte1 x1
left join cte21 x2 on x2.fa = x1.fa
with cte1 as (
/*測試資料1*/
select 1 as fa,
'a' as fb,
'c' as fc
union all
select 2 as fa,
'b' as fb,
'd' as fc
union all
select 3 as fa,
'c' as fb,
'x' as fc
), cte2 as (
/*測試資料2*/
select 1 as fa,
'c' as fb
union all
select 2 as fa,
'no' as fb
union all
select 3 as fa,
'x' as fb
), cte21 as (
/*1）新增存在標準*/
select a.*,
(case when (select count(1
) from cte1 x where x.fc = a.fb)
>
0 then 1
else
0 end) as fexist
from cte2 a
), ctepout as (
/*2)關聯，不存在的顯示為null*/
select x1.fa,
x1.fb,
x1.fc,
(case when x2.fexist =
1 then x2.fa
else null
end) as fa2,
(case when x2.fexist =
1 then x2.fb
else null
end) as fbc
from cte1 x1
left join cte21 x2 on x2.fa = x1.fa
)/*輸出結果*/
select *
from ctepout;
/*2)關聯，不存在的顯示為null*/
select x1.fa,
x1.fb,
x1.fc,
(case when x2.fexist =
1 then x2.fa else null end) as fa2,
(case when x2.fexist =
1 then x2.fb else null end) as fb2
from cte1 x1
left join (
select a.*,
(case when (select count(1
) from cte1 x where x.fc = a.fb)
>
0 then 1
else
0 end) as fexist
from cte2 a
) x2 on x2.fa = x1.fa
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