Database
How does mysql query the data of a field in the database?
How to query a certain field in the database in mysql: use the SELECT statement, use the “SELECT FROM ;” syntax to query; if you want to query multiple fields, use different field names Separate them with a comma “,”. In MySQL, you can use the SELECT statement to query data. Querying data refers to using different query methods to obtain different data from the database according to requirements, which is the most frequently used and most important operation. The syntax of SELECT is as follows: SELECT {* | } [ FROM , … [WHERE [GROUP BY [HAVING [{ }…]] [ORDER BY ] [LIMIT[,] ] ] Among them, the meaning of each clause is as follows: {*| } A field list containing asterisk wildcards, indicating the name of the field to be queried. , …, Table 1 and Table 2 indicate the source of query data, which can be single or multiple. WHERE is optional, if you select this item, the query data must meet the query condition. GROUP BY, this clause tells MySQL how to display the queried data and group it by the specified field. [ORDER BY], this clause tells MySQL in what order to display the queried…
How does mysql query two tables?
Mysql two-table query method: 1. Use “select field list from table 1, table 2 [where condition]” to query; 2. Use “SELECT field list FROM table 1 keyword JOIN table 2 ON Table1.Field = Table2.Field;” to query. How does mysql query two tables? The following article will introduce to you the method of multi-table query in mysql. There is a certain reference value, and friends in need can refer to it, and I hope it will be helpful to everyone. Multi-table joint query #Create table and data # create department CREATE TABLE IF NOT EXISTS dept ( did int not null auto_increment PRIMARY KEY, dname VARCHAR(50) not null COMMENT ‘Department Name’ )ENGINE=INNODB DEFAULT charset utf8; #Add department data INSERT INTO `dept` VALUES ('1', 'teaching department'); INSERT INTO `dept` VALUES ('2', 'Sales Department'); INSERT INTO `dept` VALUES ('3', 'Marketing Department'); INSERT INTO `dept` VALUES ('4', 'Personnel Department'); INSERT INTO `dept` VALUES ('5', 'Dept of Encouragement'); — Creator DROP TABLE IF EXISTS `person`; CREATE TABLE `person` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(50) NOT NULL, `age` tinyint(4) DEFAULT '0', `sex` enum('Male','Female','Shemale') NOT NULL DEFAULT 'Shemale', `salary` decimal(10,2) NOT NULL DEFAULT '250.00', `hire_date` date NOT NULL, `dept_id` int(11) DEFAULT NULL, PRIMARY KEY (`id`)…
How to modify mysql database table?
How to modify the mysql database table: Use the “ALTER TABLE” statement to change the structure of the original table, such as adding or deleting fields, modifying the original field data type, renaming the field or table, modifying Table character set, etc.; syntax “ALTER TABLE [modify options]”. The premise of modifying the data table is that the table already exists in the database. Modifying a table refers to modifying the structure of an existing data table in the database. The operation of modifying the data table is also indispensable in database management. Just like drawing a sketch, you can use an eraser to erase too much drawing, and add it with a pen if you have less drawing. If you don’t know how to modify the data table, it is equivalent to throwing away and redrawing as long as you make a mistake, which increases unnecessary costs. In MySQL, you can use the ALTER TABLE statement to change the structure of the original table, such as adding or deleting columns, changing the original column type, renaming columns or tables, etc. The syntax format is as follows: ALTER TABLE [modify option] modify option The syntax format is as follows: { ADD…
How does mysql clear and delete tables?
The method of clearing the table in mysql: use the “TRUNCATE table name” statement to completely clear a table; the method of deleting the table: use “DROP TABLE table name 1 [ , table name 2, table name 3 . ..];” statement. mysql empty table MySQL provides DELETE and TRUNCATE keywords to delete the data in the table. The TRUNCATE keyword is used to completely empty a table. Its syntax format is as follows: TRUNCATE [TABLE] table name The TABLE keyword can be omitted. Example Create a new table tb_student_course, insert data and The query, SQL statement and running result are as follows: mysql> CREATE TABLE `tb_student_course` ( -> `id` int(4) NOT NULL AUTO_INCREMENT, -> `name` varchar(25) NOT NULL, -> PRIMARY KEY (`id`) -> ); Query OK, 0 rows affected (0.04 sec) mysql> INSERT INTO tb_student_course(name) VALUES ('Java'),('MySQL'),('Python'); Query OK, 3 rows affected (0.05 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> SELECT * FROM tb_student_course; +—-+——–+ | id | name | +—-+——–+ | 1 | Java | | 2 | MySQL | | 3 | Python | +—-+——–+ 3 rows in set (0.00 sec) Use the TRUNCATE statement to clear the records in the tb_student_course table, the SQL statement…
