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- How to use decimal in mysql
- The DECIMAL data type in MySQL is used to represent precise decimal and monetary values. Its features include: accurately storing decimals and not being affected by rounding errors of floating point data. The range depends on the precision value, for example DECIMAL(10, 2) ranges from -9999999999.99 to 9999999999.99. More efficient than floating point data types because fewer bits are stored.
- Mysql Tutorial 827 2024-04-29 03:30:28
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- How to set composite primary key in mysql
- Methods for setting composite primary keys in MySQL include: specifying or using the ALTER TABLE statement when creating a table, which is used to uniquely identify each row and improve query performance and data integrity. Advantages include finding rows quickly, enforcing uniqueness, and maintaining uniqueness across multiple columns, but the disadvantages are increased storage overhead and performance impact.
- Mysql Tutorial 569 2024-04-29 03:27:14
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- How to set null in mysql
- Methods of setting NULL in MySQL include: 1. Use the INSERT statement to insert a NULL value; 2. Use the UPDATE statement to update the value to NULL; 3. Use the ALTER TABLE statement to set the column to allow NULL; 4. Use the DEFAULT NULL keyword to create a new table Set the column's default value to NULL. Note that NULL, unlike the empty string or zero, is always false when compared, and should be used with caution to avoid data integrity issues.
- Mysql Tutorial 500 2024-04-29 03:24:14
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- How to set two primary keys in mysql
- There are two ways to set two primary keys in MySQL: composite primary key: ALTER TABLE table_name ADD PRIMARY KEY (column1, column2); multi-column primary key: ALTER TABLE table_name ADD PRIMARY KEY USING INDEX (index_name);
- Mysql Tutorial 965 2024-04-29 03:21:15
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- What is mysql Chinese and foreign code?
- Foreign key is a data integrity constraint in MySQL, which ensures that the data of the sub-table is consistent with the main table by referencing the primary key column of the main table. The working principle of foreign code is as follows: ensure that the main table data referenced by the subtable exists. Cascade deletion: When the main table record is deleted, the related records of the sub-table are deleted in cascade. Cascade update: When the primary key of the main table is updated, the related records of the sub-table are cascade updated.
- Mysql Tutorial 314 2024-04-29 03:18:15
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- How to create a library database in mysql
- To create the MySQL database "library", please first establish a MySQL connection, and then perform the following steps: 1. Create database: CREATE DATABASE library; 2. Select database: USE library; 3. Create table: CREATE TABLE books (...); 4. Insert data: INSERT INTO books (title, author) VALUES ('...'); 5. Verify database: SHOW DATABASES; SHOW TABLES;
- Mysql Tutorial 607 2024-04-29 03:15:21
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- The only index keyword in mysql is
- The unique index keyword is UNIQUE, which is used to ensure that a specified column or combination of columns in the table has unique values. The benefits of creating a unique index include: data integrity, query performance optimization, and unique constraint enforcement.
- Mysql Tutorial 803 2024-04-29 03:12:14
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- How to use group by in sql
- The GROUP BY statement is used to group a data set by specified columns and aggregate data from the same group. Syntax: SELECT column name 1, column name 2, ...FROM table name GROUP BY grouping column name; it can be used in conjunction with aggregate functions, such as SUM, COUNT, AVG, to summarize data within a group. Benefits include simplifying data analysis, identifying pattern trends, and improving query performance.
- SQL 605 2024-04-28 21:15:25
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- What does if mean in sql
- The IF keyword in SQL is used to perform different actions based on conditions. The syntax is: IF condition THEN code_block_1 [ELSE code_block_2]. IF statements can check whether a value is empty, compare values for equality, and perform different operations based on conditions.
- SQL 479 2024-04-28 12:21:15
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- What does from dual mean in sql
- In SQL, the FROM DUAL statement retrieves one row and one column of data from a special virtual table, DUAL, that contains only one row with the value X. Common uses include: initialization sequences, inserting default values, and as a virtual table in subqueries.
- SQL 1240 2024-04-28 12:18:14
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- What does from mean in sql
- The FROM keyword is used to specify the data source (i.e. table) to be queried. Its syntax is: FROM table_name. Specify the table to be queried in the SELECT statement; specify the table to be connected in the JOIN operation; specify the table to be queried in the subquery. subtable.
- SQL 803 2024-04-28 12:15:22
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- What operation is used to implement from in sql?
- In SQL, the FROM operator is used to specify the data source, that is, the table or view from which data is to be retrieved. It is a required clause followed by the name of the table or view, which can be referenced using an alias. The WHERE clause is used to further filter the data and retrieve only rows that meet specific conditions. The FROM operator is crucial for SQL queries because it defines the data set, allows joining data, and provides the possibility to filter the data.
- SQL 708 2024-04-28 12:12:14
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- Usage of if statement in sql
- The SQL IF statement executes different queries based on conditions. The syntax is as follows: IF (condition)statement1ELSEstatement2 where condition is the condition, statement1 is the statement executed when the condition is true, and statement2 is the statement executed when the condition is false. Nested IF statements can handle more complex conditions, and IF statements can be used with other SQL statements.
- SQL 412 2024-04-28 12:09:14
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- Usage of case when statement in sql
- The CASE WHEN statement is used in SQL to evaluate an expression based on specified conditions and return a different value. Its syntax consists of WHEN clause (conditional expression and return expression), ELSE clause (default expression) and END keyword. It can be used in a variety of scenarios, including assigning values, modifying results, and converting data formats.
- SQL 677 2024-04-28 12:06:15
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- Usage of isnull function in sql
- The ISNULL() function in SQL checks whether an expression is NULL and returns the specified value. Usage is as follows: checks for a NULL value and returns a replacement value. Prevent divide-by-zero errors. Merge two fields to avoid duplication.
- SQL 651 2024-04-28 12:03:13
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