Source: San
cloud.tencent.com/developer/article/1475146
Yes, Xiao Bian blowing a cow, is this small series do not look at the tutorial will own to where 1 = 1 in the native sql plus years, and wrote the bulk insert sql assembled students should be more clear, automatic splicing
Of course, Xiao Bian also not a lot of, we need to learn and progress together
This code should be by a program (e.g., Java) generated, where conditions after condition 1 = 1 if the block is by dynamic change. E.g:
String sql=select * from table_name where 1=1;
if( conditon 1) {
sql=sql+ and var2=value2;
}
if(conditon 2) {
sql=sql+ and var3=value3;
}
where 1 = 1 is to avoid the first word directly behind where the keyword is "and" caused a syntax error.
Dynamic SQL connection AND condition
where 1 = 1 is to avoid the first word directly behind where the keyword is "and" caused a syntax error.
Where there must be a statement later, added after 1 = 1 can not go wrong syntax guarantee!
select * from table where 1=1
Because the table is simply not the name for the field 1, it is equivalent to the SQL select * from table,
The SQL statement is obviously full table scan, requires a lot of IO operation, the greater the amount of data to slower,
Proposal to increase the query losing items, that is, where 1 = 1 appends some common mandatory conditions and establish appropriate index these mandatory conditions will greatly improve efficiency
Copy table
create table table_name as select * from Source_table where 1=1;
Copy Table Structure
create table table_name as select * from Source_table where 1 <> 1;
