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What does column in mysql mean?

A column (Column) in a MySQL table is a vertically arranged data unit that contains specific data types and attributes, such as name, data type, constraints, and default values. These properties define the characteristics of the column and the way it is used to store and organize data in the table.

Apr 27, 2024 am 07:15 AM

mysql排列

Can column be omitted in mysql?

Column names cannot be omitted in MySQL. Column names are used to identify, reference data, create indexes, be used in stored procedures, and maintain data integrity. If you do not specify a column name, MySQL will throw an error. It is recommended to always give columns clear and meaningful names.

Apr 27, 2024 am 07:12 AM

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Is no a keyword in mysql?

No, NO is not a keyword in MySQL. Keywords in MySQL are predefined reserved words with specific grammatical meanings, and NO does not belong to them. It can be used freely as an identifier, such as a table name, column name, or variable name.

Apr 27, 2024 am 06:54 AM

mysql

What is the function used to query results in mysql?

Functions used in MySQL to query results include: Aggregation functions: Calculate sum, average, minimum, maximum, or number of rows. String functions: concatenate, extract, replace or get the length of a string. Mathematical functions: take absolute value, round, round up or down. Date and time functions: get date and time, extract date or time part. Other functions: get the first non-NULL value, compare expressions for equality, get the maximum or minimum value, etc.

Apr 27, 2024 am 06:51 AM

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What is the command to update table data in mysql?

The command used in MySQL to update table data is UPDATE. The update syntax is: UPDATE table_name SET column1 = value1, column2 = value2, ... WHERE condition; where condition specifies the update condition, and only rows that meet the condition are updated. Other considerations include using the WHERE clause to update all rows if no condition is specified, using indexed columns to improve performance, UPDATE to return the number of rows affected, and syntax updates to UPDATE ... SET ... RETURNING *; Data and return

Apr 27, 2024 am 06:48 AM

mysql

What is the relationship between primary key constraints and unique constraints in mysql

The relationship between primary key constraints and unique constraints in MySQL: Primary key constraints require column values to be unique and non-null; unique constraints allow null values but prevent duplicate rows.

Apr 27, 2024 am 06:45 AM

mysql

How to distinguish primary key constraints and unique constraints in mysql

The difference between a primary key and a unique constraint: the primary key forces each row of data to be unique and non-null, and is used to identify the row; the unique constraint only enforces the uniqueness of a specific column or column combination, allowing other columns to be repeated.

Apr 27, 2024 am 06:42 AM

mysql

What does enum mean in mysql

ENUM is an enumeration data type in MySQL that is used to limit the range of stored values. It works by storing a predefined list of values, ensuring data integrity, space efficiency, and performance. Creating a table with ENUM columns requires the use of the CREATE TABLE statement, and data can then be inserted and updated using predefined values. ENUM columns can be used in queries and comparisons to ensure that the data fits within the desired range.

Apr 27, 2024 am 06:39 AM

mysql

What does before mean in mysql

The BEFORE clause in MySQL performs operations before triggering events and is used to verify data, modify data, trigger event notifications, etc. The syntax is: CREATE TRIGGER trigger_name BEFORE event_type ON table_name FOR EACH ROW BEFORE [statements]. Example trigger to verify validity before inserting a record: IF NEW.salary < 10000 THEN SET NEW.salary = 10000; END IF;

Apr 27, 2024 am 06:36 AM

mysql

What does on mean in mysql

In MySQL, ON is used to join tables, and the join condition is the comparison of columns between tables. It is different from WHERE, which is used to filter query results.

Apr 27, 2024 am 06:33 AM

mysql

How to find percentage in mysql

Finding percentages in MySQL: Determine numerator and denominator. Divide the numerator by the denominator to find the result. Use the ROUND() function to round the result to a specified number of decimal places (usually two decimal places). Syntax: ROUND((numerator / denominator) * 100, 2)

Apr 27, 2024 am 06:30 AM

mysql

What does asc mean in mysql

ASC in MySQL stands for Ascending Sort and can be used to sort the data in the result set from the smallest value to the largest value.

Apr 27, 2024 am 06:27 AM

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What does where in mysql mean?

The WHERE clause in MySQL is used to filter query results and only return records that meet specified conditions. It limits the results through conditions, which consist of column names, comparison operators, values or variables, and logical operators. The WHERE clause is mainly used to retrieve records that meet conditions, exclude records that do not meet conditions, and create subsets required for joins and subqueries.

Apr 27, 2024 am 06:24 AM

mysql

What does varchar in mysql mean?

VARCHAR is a data type used in MySQL to store variable-length strings. Its features include: flexible storage of characters of different lengths; the maximum length is predefined, and the actual length is allocated according to the data; saving space and improving query performance; index efficiency and fragmentation may be affected; suitable for variable lengths where the length does not reach the maximum allowed value String.

Apr 27, 2024 am 06:21 AM

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