Quick Start with SQL Language 1_PHP Tutorial

SQL is the abbreviation of English Structured Query Language, which means structured query language. The main function of the SQL language is to establish connections and communicate with various databases. According to ANSI (American National Standards Institute), SQL is used as the standard language for relational database management systems. SQL statements can be used to perform a variety of operations, such as updating data in the database, extracting data from the database, etc. At present, most popular relational database management systems, such as Oracle, Sybase, Microsoft SQL Server, Access, etc., all adopt the SQL language standard. Although many databases have redeveloped and extended SQL statements, standard SQL commands including Select, Insert, Update, Delete, Create, and Drop can still be used to complete almost all database operations. Next, we will introduce the basic knowledge of SQL language in detail.

Database tables
A typical relational database usually consists of one or more objects called tables. All the data or information in the database is saved in these database tables. Each table in the database has its own unique table name, which is composed of rows and columns. Each column includes the column name, data type, and other attributes of the column, and the row specifically contains the information of a certain column. records or data. Below, is an example of a database table named Weather.
The highest and lowest temperature in the city
Beijing 10 5
Shanghai 15 8
Tianjin 8 2
Chongqing 20 13

In this table, "city", "maximum temperature" and "Minimum temperature" is three different columns, and each row in the table contains specific table data.

Data query
Among the many SQL commands, the select statement should be considered the most frequently used. The Select statement is mainly used to query the database and return result data that meets the user's query criteria. The syntax format of the Select statement is as follows:
select column1 [, column2, etc] from tablename
[where condition];
([] indicates optional options)
in the select statement after the select keyword Column names are used to determine which columns will be returned as query results. Users can select any column according to their needs, and can also use the wildcard "*" to set all columns in the returned table.

The table name after the from keyword in the select statement is used to determine the target table for the query operation.
The where optional clause in the Select statement is used to specify which data values ​​or rows will be returned or displayed as query results.
You can use the following operators in the where conditional clause to set query criteria:
= equal to
> greater than
>= greater than or equal to
is not equal to
In addition to the operators mentioned above, the LIKE operator is also very important in the where conditional clause. The LIKE operator is very powerful. By using the LIKE operator, you can select only records with the same format as specified by the user. In addition, we can also use the wildcard "%" to replace any string. For example:
select firstname, lastname, city
from employee
where firstname LIKE 'E%';
(note that the string must be enclosed in single brackets)
The above SQL statement All names starting with E will be searched. Or, use the following statement:
select * from employee
where firstname = ‘May’;
Query all rows named May.

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