比较两个表的最快方法是什么？ （DBMR3010）
“EXCEPT”运算符用于合并两个“SELECT”语句，并从第一个SELECT语句返回第二个SELECT语句未返回的列。 DBMaker不支持EXCEPT运算符。 因此，我们提供了一种替代解决方法。 但是，该命令仅比较两栏，而不能比较BLOB数据。
该命令代替“表格t1，表格t2除外”(“table t1 except table t2”)。
select * from t1 where not exists(select 0 from t2 where t1.c1 = t2.c1 and (t1.c2 = t2.c2 or (t1.c2 is null and t2.c2 is null)));
该命令代替“表格t2，表格t1除外”(“table t2 except table t1”)
select * from t2 where not exists(select 0 from t1 where t1.c1 = t2.c1 and(t1.c2 = t2.c2 or (t1.c2 is null and t2.c2 is null)));
例如，有两个表格，t1和t2。
Table t1
C1 C2
======= =======
1 1
2 2
3 3
Table t2
C1 C2
======= =======
1 1
2 2
4 4
执行第一个命令替换“ t1除了t2”(“t1 except t2”)。 您可以在t1中获得3，但不在t2中。
dmSQL> select * from t1 where not exists(select 0 from t2 where t1.c1 = t2.c1 and (t1.c2 = t2.c2 or (t1.c2 is null and t2.c2 is null)));
C1 C2
======= =======
3 3
======= =======
3 3
执行第二个命令替换“除了t1之外的t2”(“t2 except t1”)。您可以在t2中获得4，但不在t1中”
dmSQL> select * from t2 where not exists(select 0 from t1 where t1.c1 = t2.c1 and(t1.c2 = t2.c2 or (t1.c2 is null and t2.c2 is null)));
C1 C2
======= =======
4 4
======= =======
4 4
Version: DBMaker 5.4.2
Product: Normal/Bundle
Platform: Windows/Linux