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SQL is a standard computer language for accessing and processing databases. Note that SQL is a language. MySQL, SQL Server, MangoDB and so on are all databases.

The SQL language was proposed by Boyce and Chamberlin in 1974 and was first implemented on the relational database system SystemR developed by IBM Company. Because of its outstanding advantages such as rich functions, convenient and flexible use, concise and easy to learn language, it is deeply welcomed by the computer industry and computer users. In October 1980, X3H2, the database committee of the American National Bureau of Standards (ANSI), approved SQL as the American standard for relational database language, and published the standard SQL in the same year. Shortly after that, ISO made the same decision.

SQL command

SQL is divided into two parts: DML and DDL. DML is a data operation language and DDL is a data definition language.

DDL statement:

CREATE DATABASE: create a new database.

ALTER DATABASE: modify the database.

CREATE TABLE: create a new table.

ALTER TABLE: change (change) database tables.

DROP TABLE: delete the table.

CREATE INDEX: create an index (search key).

DROP INDEX: delete the index.

DML statement:

INSERT INTO: inserts data into a database table.

SELECT: get data from a database table.

UPDATE: update the data in the database table.

DELETE: deletes data from the database table.

INSERT INTO

The INSERT INTO statement is mainly used to insert new records into the table.

The syntax is as follows:

INSERT INTO table_nameVALUES (value1,value2,value3,...); or INSERT INTO table_name (column1,column2,column3,...) VALUES (value1,value2,value3,...); the contents of the Test_table table are as follows: idnameagecity1Mark17New York2Tom18Paris3Anni17Sydney example:

Insert a new row of data into the Test_ table table:

INSERT INTO Test_table VALUES (4 legendary CoCoke page2, 20 pas)

The output is as follows:

Query OK, 1 row affected (0.20 sec)

Insert multiple rows of new data into the Test_ table:

INSERT INTO Test_tableVALUES (5 recollective KeKeKey1 19 pyrrhic London'), (6 pyrrhic Nacify20 pyrrhenney`); (7 pyrrine Anglepique 17 pas')

The output is as follows:

Query OK, 3 rows affected (0.01sec) Records: 3 Duplicates: 0 Warnings: 0

View the data in the Test_ table

+-+ | id | name | age | city | +-- + | 1 | Mark | 17 | New York | | 2 | Tom | 18 | Paris | 3 | Anni | 17 | Sydney | 4 | KeKe | 20 | Paris | | 5 | CoCo | 19 | London | | 6 | Naci | 20 | Sydney | | 7 | Angle | 17 | Paris | +-+ SELECT |

The SELECT statement is mainly used to select data from a table. The returned data is stored in the result set.

The syntax is as follows:

SELECT column1, column2,... For example, FROM table_name;:

Query all data in the Test_ table:

SELECT * FROM Test_table

Output:

+-+ | id | name | age | city | +-- + | 1 | Mark | 17 | New York | | 2 | Tom | 18 | Paris | 3 | Anni | 17 | Sydney | 4 | KeKe | 20 | Paris | | 5 | CoCo | 19 | London | | 6 | Naci | 20 | Sydney | | 7 | Angle | 17 | Paris | +-+ |

Query the id and name fields in the Test_table table:

SELECT id, name FROM Test_table

Output:

+-+ | id | name | +-+-+ | 1 | Mark | | 2 | Tom | | 3 | Anni | | 4 | KeKe | | 5 | CoCo | | 6 | Naci | 7 | Angle | +-+-+ UPDATE

The UPDATE statement is used to modify records that already exist in the table.

The syntax is as follows:

UPDATE table_name SET column1 = value1, column2 = value2,. WHERE condition; for example:

Update the field in the table with an id of 5 and the corresponding age to 21:

UPDATE Test_table SET age=21 WHERE id=5

Output:

Query OK, 1 row affected (0.20 sec) Rows matched: 1 Changed: 1 Warnings: 0

If you want to change all the values of a field, as long as you don't specify the WHERE clause, there are no examples here. If you want to do this, remember to be careful. After all, once the data is modified, you can't go back on it.

DELETE

The DELETE statement is used to delete records from a table.

The syntax is as follows:

For example, DELETE FROM table_name WHERE condition;:

Delete data from the table with an id field value of 3:

DELETE FROM Test_table WHERE id=3

Output:

Query OK, 1 row affected (0.04 sec)

View the Test_ table:

+-- + | id | name | age | city | +-- + | 1 | Mark | 17 | New York | | 2 | Tom | 18 | Paris | 4 | KeKe | 20 | Paris | | 5 | CoCo | 19 | London | | 6 | Naci | 20 | Sydney | | 7 | Angle | 17 | Paris | +-- + |

If you do not specify the WHERE clause, you delete all records in the table. This operation should be done with special care, because once executed, the data of the entire table is actually deleted.

This is not necessary to delete the data table, after deleting the data table, the whole table no longer exists. After deleting the table record using DELETE, the table still exists, and the structure, attributes and indexes of the table will remain unchanged.

Thank you for reading! This is the end of this article on "how to add, delete, modify and search SQL". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, you can share it out for more people to see!

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