Work often encounter insert a data, which may contain a fixed value, it may be the result of the query another table
to solve the problem of two cases:
1, table2 when there is:
insert into table1 (x1,x2) values('a',(select x from table2 where x=x))
2, when the absence of table2:
insert into table1(x1,x2) select 'a' , x from table1 where where x=x
Reproduced in: https: //www.jianshu.com/p/a9304c4c940d