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There aren't many requests for database-related work, but it's not.At that time, it is too bad to throw the whole thing to the engineer.So, I picked it up to get the minimum knowledge (although I have learned in the past and have done some actual work, but I haven't learned enough to use it in practice).
What is a database and what can and cannot be done?This is essential knowledge not only for databases but also for directing any requirements.
(1) Add new ones (database area, table, record)
(2) Erase (database area, table, record)
(3) Overwrite (record)
(4) Take out (record)
(Omitted)
In SQL statements, various conditions are added when instructing.The result is a lot of SQL statements because the conditions can be complicated, but these are the only four basic things you can actually do.
A DBMS is software for operating a database, and a program gives instructions to the DBMS.It can be said that it is unlikely that any one of these three will be missing because the database system is composed of the trinity.
"NULL" is different from "0 (zero)". "Nothing" is the same, but "NULL" means that it is blank, and "0" means that there is a value of "0".