Function: The overlay function in postgresql is used to replace characters at a specific position in a string field
语法：overlay(string1 placing string2 from int for int)
string1: the name of the field to be replaced;
string2: the character to replace;
The first int: the starting position of the replacement ( note: the starting position of this method should start from 1 );
The second int: the length to be replaced
Usage: For example, if there is a field whose type is a string, and then the value is "SLP00 0 2336306", the corresponding field name in the table is slip_no, and you want to change it to "SLP00 1 2336306"
The sql statement can be written as select overlay(slip_no placing '1' from 6 for 1) as slip_no from table (table name)