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Technical Articles

Database

SQL

What is the function of from in sql

The role of the FROM clause in SQL is to specify the data source, including: retrieving data from one or more tables, establishing association conditions between tables, and filtering data in conjunction with the WHERE clause.

May 01, 2024 pm 09:48 PM

The statement to implement data retrieval in sql is

The SQL statement used for data retrieval is SELECT. The syntax includes SELECT (specify columns), FROM (specify tables), and WHERE (specify conditions). Examples include retrieving all student names and ages, retrieving students older than 18, and retrieving all information for a specific student.

May 01, 2024 pm 09:48 PM

What does like mean in sql

LIKE is an operator in SQL that searches a table for rows that match a specified pattern. It uses the wildcard characters % and to match characters, where % matches any number of characters and matches any single character.

May 01, 2024 pm 09:48 PM

What does where1=1 mean in sql?

WHERE 1=1 is a placeholder in a SQL query that means it is always true. It is used to ensure that queries always return data, simplify queries, and prevent errors due to missing WHERE clauses.

May 01, 2024 pm 09:45 PM

In sql, which one is executed first, where or on?

The execution order of WHERE and ON clauses is: 1. The WHERE clause is executed first and rows that meet the conditions are filtered out. 2. The ON clause is then applied to the filtered rows, establishing a join based on the join conditions.

May 01, 2024 pm 09:42 PM

The difference between where and having in sql

The WHERE and HAVING clauses are both used to filter data in SQL, but their scopes are different: the WHERE clause filters single rows, and the HAVING clause filters the grouped result set. The WHERE clause is applied before grouping and affects the result rows of the aggregate function; the HAVING clause is applied after grouping and affects grouped rows rather than individual rows. The WHERE clause can filter any column, while the HAVING clause can only filter the results of aggregate functions.

May 01, 2024 pm 09:42 PM

聚合函数

What does drop mean in sql?

The DROP command in SQL is used to delete database objects (such as tables, views, indexes), including the following uses: Delete objects that are no longer needed or are obsolete. Delete incorrect or damaged objects. Modify the database structure.

May 01, 2024 pm 09:42 PM

What operation is used to implement where in sql?

The WHERE statement is used to add filter conditions in the SQL query to select only records that meet the specified conditions. The syntax is: SELECT column name FROM table name WHERE condition. The WHERE statement uses various conditional operators, including: =, <>, >, >=, <, <=, BETWEEN, LIKE, IN. Connectors (AND, OR, NOT) can be used to combine multiple conditions.

May 01, 2024 pm 09:40 PM

The role of from in sql

The FROM clause in SQL is used to specify the data source of the query, including: Specify the data source: Define the table or view to be queried. Join tables: Combine data from multiple tables into a query using the JOIN keyword. Set query scope: retrieve only data in the specified table or view.

May 01, 2024 pm 09:39 PM

作用域

What does ‖ mean in sql

The | operator in SQL represents a logical OR operation that joins two Boolean values and returns a Boolean value: if both operands are TRUE, the result is TRUE. If both operands are FALSE, the result is FALSE. If one operand is TRUE and the other is FALSE, the result is TRUE. It is often used to combine conditions in the WHERE clause. It has lower priority and requires careful use of parentheses.

Apr 29, 2024 pm 04:30 PM

What does modify mean in sql

The MODIFY command in SQL is used to modify database data or schema, including: Modify data: Update or delete rows in an existing table. Modify the schema: add or remove columns, change column types or constraints, etc.

Apr 29, 2024 pm 04:27 PM

What does unique mean in mysql

The unique constraint in MySQL ensures that the value in a column or column group is unique and prevents duplicate values. It is implemented by creating an index, which can enhance data integrity, query optimization, and data consistency.

Apr 29, 2024 pm 04:15 PM

mysql

what does union mean in sql

The UNION operator is used in SQL to merge rows with the same structure, producing a table containing all unique rows. It works by merging result sets, removing duplicate rows and returning remaining rows. Unlike UNION ALL, UNION returns only unique rows. When using UNION, note that both tables must have the same number of columns and data types.

Apr 29, 2024 pm 04:03 PM

What does asterisk mean in sql

The asterisk (*) in SQL is a wildcard character used to retrieve all columns in a table. It can be used to retrieve all columns or columns with a specific prefix: Retrieve all columns: SELECT * FROM table_name; Retrieve columns with a specific prefix: SELECT customer_* FROM table_name;

Apr 29, 2024 pm 03:54 PM