[原创]以下ＳＱＬ语句的格式是否正确？

close;
SQL.clear;
sql.add('insert into 客户信息表');
sql.add('values("'+edit3.text+'","'+edit4.Text+'","'+edit9.Text+'","'+edit5.Text+'","'+edit6.text+'","'+edit10.Text+'","'+edit14.Text+'","'+edit15.Text+'","'+edit11.Text+'","'+edit12.Text+'","'+edit13.Text+'","'+edit29.Text+'","'+edit8.Text+'","'+edit7.Text+'","'+edit19.Text+'","'+edit21.Text+'","'+edit17.Text+'","'+edit20.Text+'","'+edit22.Text+'","'+edit23.Text+'","'+edit24.Text+'","'+edit27.Text+'","'+edit26.Text+'","'+edit28.Text+'")');
execsql;
close;
sql.clear;
sql.add('select * from 客户信息表');
open;
想问"'+edit4.Text+'"里的＂＋＂起什么作用？还有就是什么时候用的单引号，什么时候用的双引号？