foreword
As a language, SQL, like other programming languages, needs to follow some specific norms and guidelines, which is what we often call syntax (Syntax).
Here are a few syntax rules for SQL:
- All SQL syntax must start with keywords (also known as commands), such as SELECT, INSERT, UPDATE, DELETE, ALTER, DROP, CREATE, USE, SHOW, etc.
- All SQL statements must end with a semicolon;. SQL statements can span lines, and you can write SQL statements in one or more lines.
- SQL is case-insensitive, which means that SELECT and select are the same in SQL statements, but keywords usually appear in uppercase.
Note: Table names are case-sensitive, you must use table names that already exist in the database.
Example SQL syntax:
SELECT column_name FROM table_name;
Commonly used SQL commands
1、SELECT
The SELECT command is used to query (select) data from the database, and its usage is as follows:
SELECT column1, column2....columnN
FROM table_name;
2、UPDATE
The UPDATE command is used to update data in the database, and its usage is as follows:
UPDATE table_name
SET column1 = value1, column2 = value2....columnN=valueN
[ WHERE CONDITION ];
3、DELETE
The DELETE command is used to delete data from the database, and its usage is as follows:
DELETE FROM table_name
WHERE {CONDITION};
4、CREATE TABLE
The CREATE TABLE command is used to create a new data table, and its usage is as follows:
CREATE TABLE table_name(
column1 datatype,
column2 datatype,
column3 datatype,
.....
columnN datatype,
PRIMARY KEY( one or more columns )
);
5、ALTER TABLE
The ALTER TABLE command is used to modify data tables. ALTER TABLE can be used to modify the fields of the data table, for example:
ALTER TABLE table_name {ADD|DROP|MODIFY} column_name {data_ype};
It can also be used to modify the name of the data table, for example:
ALTER TABLE table_name RENAME TO new_table_name;
6、DROP TABLE
The DROP TABLE command is used to delete a data table, and its usage is as follows:
DROP TABLE table_name;
7、CREATE DATABASE
CREATE DATABASE is used to create a new database, its usage is as follows:
CREATE DATABASE database_name;
8、INSERT INTO
The INSERT INTO command inserts new data into the database, and its usage is as follows:
INSERT INTO table_name( column1, column2....columnN)
VALUES ( value1, value2....valueN);
9、CREATE INDEX
The CREATE INDEX command is used to create an index, and its usage is as follows:
CREATE UNIQUE INDEX index_name
ON table_name ( column1, column2,...columnN);
10、DROP INDEX
The DROP INDEX command is used to delete an index, and its usage is as follows:
ALTER TABLE table_name
DROP INDEX index_name;
Summarize
This article mainly explains the common syntax of SQL, including various operations of adding, deleting, modifying and checking. We will introduce related operations of SQL database later.
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