? 為了更加深入左外連接，我們做一些測試，外連接的寫法有幾種形式，我們能夠通過10053跟蹤到終于SQL轉換的形式。

--初始化數據

create table A

(
? id number,
? age number
);
create table b
(
? id number,
? age number
);
insert into A values(1,10);
insert into A values(2,20);
insert into A values(3,30);
insert into B values(1,10);
insert into B values(2,20);
commit;

--用10053找到終于轉換后的SQL

alter session set session_cached_cursors =0;
alter session set events '10053 trace name context forever, level ?1';
explain plan for select * from A left join B on A.id = B.id and A.age > 5;
explain plan for select * from A left join B on A.id = B.id WHERE A.age > 5;
explain plan for select * from A left join B on A.id = B.id and b.age > 5;
explain plan for select * from A left join B on A.id = B.id where b.age > 5;
alter session set events '10053 trace name context off' ;


select * from A left join B on A.id = B.id and A.age > 5;
? ? ? ? ID ? ? ? ?AGE ? ? ? ? ID ? ? ? ?AGE
---------- ---------- ---------- ----------
? ? ? ? ?1 ? ? ? ? 10 ? ? ? ? ?1 ? ? ? ? 10
? ? ? ? ?2 ? ? ? ? 20 ? ? ? ? ?2 ? ? ? ? 20
? ? ? ? ?3 ? ? ? ? 30
--Final query after transformations:
SELECT "A"."ID" "ID", "A"."AGE" "AGE", "B"."ID" "ID", "B"."AGE" "AGE"
? FROM "GG_TEST"."A" "A", "GG_TEST"."B" "B"
?WHERE "A"."ID" = "B"."ID"(+)
? ?AND "A"."AGE" > CASE WHEN("B"."ID"(+) IS NOT NULL) THEN 5 ELSE 5 END


select * from A left join B on A.id = B.id WHERE A.age > 5;
? ? ? ? ID ? ? ? ?AGE ? ? ? ? ID ? ? ? ?AGE
---------- ---------- ---------- ----------
? ? ? ? ?1 ? ? ? ? 10 ? ? ? ? ?1 ? ? ? ? 10
? ? ? ? ?2 ? ? ? ? 20 ? ? ? ? ?2 ? ? ? ? 20
? ? ? ? ?3 ? ? ? ? 30
--Final query after transformations:
SELECT "A"."ID" "ID", "A"."AGE" "AGE", "B"."ID" "ID", "B"."AGE" "AGE"
? FROM "GG_TEST"."A" "A", "GG_TEST"."B" "B"
?WHERE "A"."AGE" > 5
? ?AND "A"."ID" = "B"."ID"(+);


select * from A left join B on A.id = B.id and b.age > 5;?
? ? ? ? ID ? ? ? ?AGE ? ? ? ? ID ? ? ? ?AGE
---------- ---------- ---------- ----------
? ? ? ? ?1 ? ? ? ? 10 ? ? ? ? ?1 ? ? ? ? 10
? ? ? ? ?2 ? ? ? ? 20 ? ? ? ? ?2 ? ? ? ? 20
? ? ? ? ?3 ? ? ? ? 30 ?
--Final query after transformations:
SELECT "A"."ID" "ID", "A"."AGE" "AGE", "B"."ID" "ID", "B"."AGE" "AGE"
? FROM "GG_TEST"."A" "A", "GG_TEST"."B" "B"
?WHERE "A"."ID" = "B"."ID"(+)
? ?AND "B"."AGE"(+) > 5

--這樣的形式你能夠看到外連接失效，CBO還是很聰明的
select * from A left join B on A.id = B.id where b.age > 5;
? ? ? ? ID ? ? ? ?AGE ? ? ? ? ID ? ? ? ?AGE
---------- ---------- ---------- ----------
? ? ? ? ?1 ? ? ? ? 10 ? ? ? ? ?1 ? ? ? ? 10
? ? ? ? ?2 ? ? ? ? 20 ? ? ? ? ?2 ? ? ? ? 20
--Final query after transformations:
SELECT "A"."ID" "ID", "A"."AGE" "AGE", "B"."ID" "ID", "B"."AGE" "AGE"
? FROM "GG_TEST"."A" "A", "GG_TEST"."B" "B"
?WHERE "B"."AGE" > 5
? ?AND "A"."ID" = "B"."ID";

轉載于:https://www.cnblogs.com/yfceshi/p/6806397.html

總結

以上是生活随笔為你收集整理的Oracle 左外连接的一些測试的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。