How do you enable error reporting in PHP?

 PHP allows us to choose whether or not to display errors to users. There are three ways to enable or disable error reporting in PHP:

Method 1: You can turn the display error parameter in the php.ini file on or off. When it’s on, errors are shown; when it’s off, there are no errors shown or reported. To do this, locate the display error parameter in the php.ini file, then set its value to “on.” Restart the web server's various services. The PHP script's errors will now be visible to end users. Set the display error parameter to “off” to stop reporting errors. The off argument indicates that users won't see any errors.

Method 2: Using the mini set() function is the second way to enable or stop error reporting. The ability to modify the configuration options in the php.ini file is a built-in feature of the PHP programming language. It requires two inputs: the name of the setting you want to change and the value you want to assign.

Method 3: You can also utilize the error reporting() function, a native PHP function, to enable error reporting. This function controls which PHP faults are reported and sets the error reporting directive during runtime. Given that PHP has multiple error levels, this method also aids in setting the various levels for the runtime (duration) of your script. This function accepts parameters for error names like E ERROR, E WARNING, E PARSE, etc., allowing these errors to be reported.

Write a query for a column addition in MySQL

 For this, an ALTER TABLE query is required. Once invoked, simply mention the column and its definition. Something like this:

ALTER TABLE cars

ADD COLUMN engine VARCHAR(80) AFTER colour;

How To Delete Duplicate Rows in MySQL

 Summary: in this tutorial, you will learn various ways to delete duplicate rows in MySQL.

In the previous tutorial, we have shown you how to find duplicate values in a table. Once the duplicates rows are identified, you may want to delete them to clean up your data.

Prepare sample data

The following script creates table contacts and inserts sample data into the contacts table for the demonstration.

DROP TABLE IF EXISTS contacts;

CREATE TABLE contacts (
    id INT PRIMARY KEY AUTO_INCREMENT,
    first_name VARCHAR(50) NOT NULL,
    last_name VARCHAR(50) NOT NULL, 
    email VARCHAR(255) NOT NULL
);

INSERT INTO contacts (first_name,last_name,email) 
VALUES ('Carine ','Schmitt','carine.schmitt@verizon.net'),
       ('Jean','King','jean.king@me.com'),
       ('Peter','Ferguson','peter.ferguson@google.com'),
       ('Janine ','Labrune','janine.labrune@aol.com'),
       ('Jonas ','Bergulfsen','jonas.bergulfsen@mac.com'),
       ('Janine ','Labrune','janine.labrune@aol.com'),
       ('Susan','Nelson','susan.nelson@comcast.net'),
       ('Zbyszek ','Piestrzeniewicz','zbyszek.piestrzeniewicz@att.net'),
       ('Roland','Keitel','roland.keitel@yahoo.com'),
       ('Julie','Murphy','julie.murphy@yahoo.com'),
       ('Kwai','Lee','kwai.lee@google.com'),
       ('Jean','King','jean.king@me.com'),
       ('Susan','Nelson','susan.nelson@comcast.net'),
       ('Roland','Keitel','roland.keitel@yahoo.com');
Code language: SQL (Structured Query Language) (sql)

Note that you can execute this script to recreate test data after you execute a DELETE statement.

How To Find Duplicate Values in MySQL

 Summary: in this tutorial, you will learn how to find duplicate values of one or more columns in MySQL.

Data duplication happens because of many reasons. Finding duplicate values is one of the important tasks that you must deal with when working with the databases.

Setting up a sample table

First, create a table named contacts with four columns: id, first_name, last_name, and email.

CREATE TABLE contacts (
    id INT PRIMARY KEY AUTO_INCREMENT,
    first_name VARCHAR(50) NOT NULL,
    last_name VARCHAR(50) NOT NULL,
    email VARCHAR(255) NOT NULL
);
Code language: SQL (Structured Query Language) (sql)

Second, inserts rows into the contacts table:

INSERT INTO contacts (first_name,last_name,email) 
VALUES ('Carine ','Schmitt','carine.schmitt@verizon.net'),
       ('Jean','King','jean.king@me.com'),
       ('Peter','Ferguson','peter.ferguson@google.com'),
       ('Janine ','Labrune','janine.labrune@aol.com'),
       ('Jonas ','Bergulfsen','jonas.bergulfsen@mac.com'),
       ('Janine ','Labrune','janine.labrune@aol.com'),
       ('Susan','Nelson','susan.nelson@comcast.net'),
       ('Zbyszek ','Piestrzeniewicz','zbyszek.piestrzeniewicz@att.net'),
       ('Roland','Keitel','roland.keitel@yahoo.com'),
       ('Julie','Murphy','julie.murphy@yahoo.com'),
       ('Kwai','Lee','kwai.lee@google.com'),
       ('Jean','King','jean.king@me.com'),
       ('Susan','Nelson','susan.nelson@comcast.net'),
       ('Roland','Keitel','roland.keitel@yahoo.com');
Code language: SQL (Structured Query Language) (sql)

What does a MySQL database contain?

 A MySQL database contains one or more tables, each of which contains records or rows. Within these rows are various columns or fields that contain the data itself.

What does SQL in MySQL stand for?

 The SQL in MySQL stands for Structured Query Language. This language is also used in other databases such as Oracle and Microsoft SQL Server.  One can use commands such as the following to send requests from a database:

SELECT title FROM publications WHERE author = ' J. K. Rowling’;

Note that SQL is not case sensitive. However, it is a good practice to write

the SQL keywords in CAPS and other names and variables in a small case.

What do you mean by ‘databases’?

 A database is a structured collection of data stored in a computer system and organized in a way to be quickly searched. With databases, information can be rapidly retrieved.

What are some of the advantages of using MySQL?

 

• Flexibility: MySQL runs on all operating systems
• Power: MySQL focuses on performance
• Enterprise-Level SQL Features: MySQL had for some time been lacking in advanced features such as subqueries, views, and stored procedures.
• Full-Text Indexing and Searching
• Query Caching: This helps enhance the speed of MySQL greatly
• Replication: One MySQL server can be duplicated on another, providing numerous advantages
• Configuration and Security

What is MySQL?

 MySQL is a database management system for web servers. It can grow with the website as it is highly scalable. Most of the websites today are powered by MySQL.

ENUM and SET Constraints

 ENUM and SET columns provide an efficient way to define columns that can contain only a given set of values. See Section 11.3.5, “The ENUM Type”, and Section 11.3.6, “The SET Type”.

Unless strict mode is disabled (not recommended, but see Section 5.1.11, “Server SQL Modes”), the definition of a ENUM or SET column acts as a constraint on values entered into the column. An error occurs for values that do not satisfy these conditions:

• An ENUM value must be one of those listed in the column definition, or the internal numeric equivalent thereof. The value cannot be the error value (that is, 0 or the empty string). For a column defined as ENUM('a','b','c'), values such as '', 'd', or 'ax' are invalid and are rejected.

• A SET value must be the empty string or a value consisting only of the values listed in the column definition separated by commas. For a column defined as SET('a','b','c'), values such as 'd' or 'a,b,c,d' are invalid and are rejected.

Errors for invalid values can be suppressed in strict mode if you use INSERT IGNORE or UPDATE IGNORE. In this case, a warning is generated rather than an error. For ENUM, the value is inserted as the error member (0). For SET, the value is inserted as given except that any invalid substrings are deleted. For example, 'a,x,b,y' results in a value of 'a,b'.

Multiple-Column Indexes

 MySQL can create composite indexes (that is, indexes on multiple columns). An index may consist of up to 16 columns. For certain data types, you can index a prefix of the column (see Section 8.3.5, “Column Indexes”).

MySQL can use multiple-column indexes for queries that test all the columns in the index, or queries that test just the first column, the first two columns, the first three columns, and so on. If you specify the columns in the right order in the index definition, a single composite index can speed up several kinds of queries on the same table.

A multiple-column index can be considered a sorted array, the rows of which contain values that are created by concatenating the values of the indexed columns.

Note

As an alternative to a composite index, you can introduce a column that is “hashed” based on information from other columns. If this column is short, reasonably unique, and indexed, it might be faster than a “wide” index on many columns. In MySQL, it is very easy to use this extra column:

SELECT * FROM tbl_name
  WHERE hash_col=MD5(CONCAT(val1,val2))
  AND col1=val1 AND col2=val2;

Suppose that a table has the following specification:

CREATE TABLE test (
    id         INT NOT NULL,
    last_name  CHAR(30) NOT NULL,
    first_name CHAR(30) NOT NULL,
    PRIMARY KEY (id),
    INDEX name (last_name,first_name)
);

The name index is an index over the last_name and first_name columns. The index can be used for lookups in queries that specify values in a known range for combinations of last_name and first_name values. It can also be used for queries that specify just a last_name value because that column is a leftmost prefix of the index (as described later in this section). Therefore, the name index is used for lookups in the following queries:

SELECT * FROM test WHERE last_name='Jones';

SELECT * FROM test
  WHERE last_name='Jones' AND first_name='John';

SELECT * FROM test
  WHERE last_name='Jones'
  AND (first_name='John' OR first_name='Jon');

SELECT * FROM test
  WHERE last_name='Jones'
  AND first_name >='M' AND first_name < 'N';

However, the name index is not used for lookups in the following queries:

SELECT * FROM test WHERE first_name='John';

SELECT * FROM test
  WHERE last_name='Jones' OR first_name='John';

Suppose that you issue the following SELECT statement:

SELECT * FROM tbl_name
  WHERE col1=val1 AND col2=val2;

If a multiple-column index exists on col1 and col2, the appropriate rows can be fetched directly. If separate single-column indexes exist on col1 and col2, the optimizer attempts to use the Index Merge optimization (see Section 8.2.1.3, “Index Merge Optimization”), or attempts to find the most restrictive index by deciding which index excludes more rows and using that index to fetch the rows.

If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to look up rows. For example, if you have a three-column index on (col1, col2, col3), you have indexed search capabilities on (col1), (col1, col2), and (col1, col2, col3).

MySQL cannot use the index to perform lookups if the columns do not form a leftmost prefix of the index. Suppose that you have the SELECT statements shown here:

SELECT * FROM tbl_name WHERE col1=val1;
SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;

SELECT * FROM tbl_name WHERE col2=val2;
SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;

If an index exists on (col1, col2, col3), only the first two queries use the index. The third and fourth queries do involve indexed columns, but do not use an index to perform lookups because (col2) and (col2, col3) are not leftmost prefixes of (col1, col2, col3).