Page 9-Mysql Knowledge Programming Tutorials: Learn Mysql Knowledge Online-php.cn

Article Tags

Mysql Knowledge

Home

Technical articles

Daily Programming

Mysql Knowledge

Are there any auto-increment constraints in mysql?

yes. Autoincrement constraints in MySQL allow the automatic generation of unique and incrementing integer values, usually stored in a special column called AUTO_INCREMENT. This constraint ensures data integrity, simplifies primary key generation, and facilitates the generation of incremental values such as sequence numbers.

Apr 27, 2024 am 02:36 AM

mysql

What are the keywords for self-increasing constraints in mysql?

MySQL's auto-increment constraint generates a unique incrementing ID value for each row through the AUTO_INCREMENT keyword. This constraint is suitable for integer data types. It can also be used in conjunction with the PRIMARY KEY (for primary key) and UNIQUE (for unique index) keywords, where the primary key cannot contain NULL values and each value must be unique, while the unique index can contain NULL values but other values must be unique.

Apr 27, 2024 am 02:33 AM

mysql

How to use decimal in mysql

The DECIMAL data type is used to store high-precision decimal numbers with a fixed number of decimal places. It is especially suitable for scenarios that require precise numerical calculations and financial operations. The syntax is DECIMAL(precision, scale), where precision specifies the total number of digits and scale specifies the number of digits to the right of the decimal point.

Apr 27, 2024 am 02:30 AM

mysql

The role of union in mysql

The UNION operator in MySQL combines result sets from multiple queries into a single result, eliminating duplicate rows and merging rows from different queries. It can simplify complex queries, improve query efficiency, and avoid duplicate data. Note that the queries must have the same number of columns and data types, the order of the result set is determined by the first query, and the UNION operator uses additional memory when executing.

Apr 27, 2024 am 02:27 AM

mysql

What does unique key mean in mysql

The unique key in MySQL is a constraint that ensures the uniqueness of a column value or column combination. It allows duplicate values but a specific combination of values can only appear once. It is mainly used to ensure data integrity, improve query efficiency and enforce business rules.

Apr 27, 2024 am 02:24 AM

mysql

Usage of key in mysql

MySQL's Key is an index used to quickly find data, speeding up queries and improving performance. It comes in different types including primary key, unique key, normal index, composite index and full text index. Keys can be created using the CREATE INDEX statement. Choosing the right Key is critical to optimizing queries, taking into account common query patterns, range searches, and table update frequency.

Apr 27, 2024 am 02:21 AM

mysql

The difference between = and in in mysql

In MySQL, the "=" operator compares two values for equality, while the "IN" operator checks whether a value is contained in a list of values. The "=" operator only compares two values, while the "IN" operator can compare one value with multiple values at a time. The "=" operator generally performs better than the "IN" operator and should be preferred if possible.

Apr 27, 2024 am 02:18 AM

mysql

What does Cno mean in mysql

Cno is the default name in MySQL representing a subquery column alias, used to refer to columns in the subquery. A subquery is a SELECT statement nested in the main query and needs to be assigned a column alias to reference the column. Cno aliases can be used to reference specific columns in a subquery, in ascending order of columns.

Apr 27, 2024 am 02:15 AM

mysql

How to use inner join in mysql

INNER JOIN is an operation that joins tables and returns only rows with matching records in both tables. It is used to correlate records from different tables, filter and obtain specific data, and create complex queries. When using it, you need to specify the connection conditions, that is, the matching columns between the two tables. This operation returns only matching rows, unmatched rows are filtered out.

Apr 27, 2024 am 02:12 AM

mysql

How to use in in mysql

MySQL's IN operator checks whether a column is contained in a specified list of values. Syntax: WHERE column_name IN (value1, value2, ..., valueN). Advantages include improved efficiency and simplified queries.

Apr 27, 2024 am 02:09 AM

mysql

The difference between in and on in mysql

The difference between MySQL IN and ON: IN is used to compare the value of a column with a list of values, ON is used to join two tables and compare columns with equality or inequality constraints.

Apr 27, 2024 am 02:06 AM

mysql

The difference between char(20 and varchar(20) in mysql

The main difference between CHAR(20) and VARCHAR(20) is the storage space allocation method: CHAR(20) is a fixed length, allocating 20 bytes; VARCHAR(20) is a variable length, allocating the length of the actual stored data, which is more efficient .

Apr 27, 2024 am 02:03 AM

mysql

The difference between varchar and char in mysql

The difference between VARCHAR and CHAR in MySQL: storage space allocation: VARCHAR is allocated dynamically, saving space; CHAR is allocated fixedly, not saving space. Padding: VARCHAR is not padded, CHAR is padded with spaces. Comparison: VARCHAR compares actual data, CHAR compares fixed length (including spaces). Index: VARCHAR indexes actual data, CHAR indexes fixed length (including spaces). Efficiency: VARCHAR insertion and update are efficient, and CHAR query is efficient. Selection principle: Storage space is important and data length varies greatly, so choose VARCHAR. Comparison sorting exact match, data length is fixed, select CHAR. create index,

Apr 27, 2024 am 02:00 AM

mysql

The relationship between group by and having in mysql

GROUP BY groups data of the same value and executes the aggregate function; HAVING filters the grouped data: sets conditions based on the aggregate function or grouping key; after GROUP BY is grouped, HAVING filters the grouping results and is only applicable to the aggregated data.

Apr 27, 2024 am 01:57 AM

mysql聚合函数