A field with a NULL value is a field with no value.
If a field in a table is optional, it is possible to insert a new record or update a record without adding a value to this field. Then, the field will be saved with a NULL value.
Note: A NULL value is different from a zero value or a field that contains spaces. A field with a NULL value is one that has been left blank during record creation!
The INSERT INTO statement is used to insert new records in a table.
It is possible to write the INSERT INTO statement in two ways:
1. Specify both the column names and the values to be inserted:
INSERT INTO table_name (column1, column2, column3, ...)
VALUES (value1, value2, value3, ...);
2. If you are adding values for all the columns of the table, you do not need to specify the column names in the SQL query. However, make sure the order of the values is in the same order as the columns in the table. Here, the
INSERT INTO
The ORDER BY keyword is used to sort the result-set in ascending or descending order.
The ORDER BY keyword sorts the records in ascending order by default. To sort the records in descending order, use the
DESC
The WHERE clause can be combined with
ANDOR, and
NOT
The AND and OR operators are used to filter records based on more than one condition:
AND operator displays a record if all the conditions separated by
ANDOR operator displays a record if any of the conditions separated by
ORThe NOT operator displays a record if the condition(s) is NOT TRUE. Continue reading MySQL AND, OR and NOT Operators
The WHERE clause is used to filter records.
It is used to extract only those records that fulfill a specified condition.
SELECT column1, column2, ... FROM table_name WHERE condition;
Note: The WHERE clause is not only used in
SELECTUPDATE, DELETE, etc.!
The SELECT statement is used to select data from a database.
The data returned is stored in a result table, called the result-set.
SELECT column1, column2, …
FROM table_name;
Here, column1, column2, … are the field names of the table you want to select data from. If you want to select all the fields available in the table, use the following syntax:
SELECT * FROM table_name;
SQL is the standard language for dealing with Relational Databases.
SQL is used to insert, search, update, and delete database records.
The following SQL statement selects all the records in the “Customers” table:
SELECT * FROM Customers;
select is the same as
SELECTIn this tutorial we will write all SQL keywords in upper-case.
Some database systems require a semicolon at the end of each SQL statement.
Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.
In this tutorial, we will use semicolon at the end of each SQL statement.
Hooks are reusable functions.
When you have component logic that needs to be used by multiple components, we can extract that logic to a custom Hook.
Custom Hooks start with “use”. Example: useFetch.
The React useMemo Hook returns a memoized value.
Think of memoization as caching a value so that it does not need to be recalculated.
The useMemo Hook only runs when one of its dependencies update.
This can improve performance.
The useMemo and useCallback Hooks are similar. The main difference is that useMemo returns a memoized value and useCallback returns a memoized function. You can learn more about useCallback in the useCallback article.
The React useCallback Hook returns a memoized callback function.
Think of memoization as caching a value so that it does not need to be recalculated.
This allows us to isolate resource intensive functions so that they will not automatically run on every render.
The useCallback Hook only runs when one of its dependencies update.
This can improve performance.
The useCallback and useMemo Hooks are similar. The main difference is that useMemo returns a memoized value and useCallback returns a memoized function. You can learn more about useMemo in the useMemo article.