Page 9-Daily Programming Programming Tutorials: Learn Daily Programming Online-php.cn

Article Tags

Daily Programming

Home

Technical articles

Daily Programming

Statements used to group in mysql

The statement used for grouping in MySQL is GROUP BY. It summarizes data by grouping by specified columns and performing aggregate functions (such as SUM, COUNT, AVG) on each group.

Apr 27, 2024 am 02:48 AM

mysql聚合函数

What is the difference between primary key and foreign key in mysql

The difference between primary key and foreign key: Function: The primary key uniquely identifies the record, and the foreign key associates records in different tables. Uniqueness: The primary key is unique and no duplication is allowed; the foreign key is not unique and can be repeated. NULL values: NULL values are not allowed for primary keys and NULL values are allowed for foreign keys. Relationship: The primary key is the relationship within the table, and the foreign key is the relationship between tables. Constraints: The primary key has non-null and unique constraints, and the foreign key has constraints that reference the primary key of the main table. Quantity: Each table can only have one primary key, and a table can have multiple foreign keys.

Apr 27, 2024 am 02:45 AM

mysql

How to express non-null in mysql

There are two ways to express non-null in MySQL: use NOT NULL constraint to prohibit fields from having null values. Use the DEFAULT value to specify a default value for fields that are not explicitly specified.

Apr 27, 2024 am 02:42 AM

mysql

The difference between decimal and double in mysql

DECIMAL and DOUBLE are floating-point data types in MySQL, mainly different in precision and range. DECIMAL has a fixed precision and does not round, while DOUBLE precision depends on the hardware and may have rounding errors. DECIMAL allows a specified range, while DOUBLE has a fixed range and can handle larger numbers but with less precision. The storage space of DECIMAL depends on the precision, and the storage space of DOUBLE is fixed. Use DECIMAL for exact calculations and fixed precision, use DOUBLE for approximate calculations, large numbers, or space constraints.

Apr 27, 2024 am 02:39 AM

mysql

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