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SQL Classic 50 Question Answers

SQL Classic 50 Question Answers

SQL (Structured Query Language) is a programming language used to create, manage, and query databases. The main functions include: creating databases and tables, inserting, updating and deleting data, sorting and filtering results, aggregating functions, joining tables, subqueries, operators, functions, keywords, data manipulation/definition/control language, connection types, query optimization, security, tools, resources, versions, common errors, debugging techniques, best practices, trends and row locking.

Apr 09, 2025 pm 01:33 PM
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How to add columns in SQL at the beginning of a table?

How to add columns in SQL at the beginning of a table?

How to add a column at the beginning of a SQL table? Use the ALTER TABLE statement and specify the location of the new column (for example, the BEFORE keyword in MySQL). For large tables, consider using more advanced tips for creating new tables and migrating data. Set appropriate default values ​​to avoid null value issues. Always conduct adequate testing in a development or test environment before performing any operations that modify the database structure.

Apr 09, 2025 pm 01:30 PM
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How to add columns in SQL at the end of a table?

How to add columns in SQL at the end of a table?

Adding columns at the end of a database table is not easy, the specific operation depends on the database system, table size and data volume. Common errors include: ignoring data types, incorrectly using indexes, and concurrent operations. Optimization strategies include: selecting the appropriate storage engine, using partition tables, and utilizing database replication technology. Good code readability and maintainability also help avoid problems. Only by operating with caution and paying attention to the underlying mechanism can we avoid risks in data security and integrity.

Apr 09, 2025 pm 01:27 PM
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How to add columns after specifying them in SQL?

How to add columns after specifying them in SQL?

In SQL, the steps to insert new columns gracefully: Create a new table with new columns and original columns, and formulate the column order as needed. Insert the old table data into the new table and specify the default value for the new column. Delete the old table and rename the new table to the original table name.

Apr 09, 2025 pm 01:24 PM
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How to add multiple columns in SQL?

How to add multiple columns in SQL?

How to add multiple columns to an SQL database: plan to add column names, types, and whether to allow empty; use transactions to batch add columns to improve efficiency and ensure data consistency; select the appropriate data type to avoid data redundancy; set appropriate default values ​​to avoid null value problems; add indexes and constraints to improve query efficiency and ensure data integrity.

Apr 09, 2025 pm 01:21 PM
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How does SQL modify the data type of added columns?

How does SQL modify the data type of added columns?

Modifying the data type of added columns requires caution, depending on the database system and the data itself. Direct modification may cause data truncation or loss. It is recommended to check the data first to ensure that the data is still within the new type range after conversion. If there is risk of truncation, a safer conversion method can be used, such as migrating data and deleting old columns. Also, you need to pay attention to the impact of format conversion rules, lock tables, etc., operate with caution in the production environment, and back up data.

Apr 09, 2025 pm 01:18 PM
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How to modify the default value of added columns in SQL?

How to modify the default value of added columns in SQL?

SQL to modify the default value of added columns cannot be modified directly. The following steps are required: Use the ALTER TABLE statement to modify the default value of newly inserted data with ALTER COLUMN. Use the UPDATE statement to update existing data and then modify the default values, but be cautious when updating large amounts of data. Consider complex situations such as data type changes, and do a good job in transaction control and backup.

Apr 09, 2025 pm 01:15 PM
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How does SQL modify constraints for added columns?

How does SQL modify constraints for added columns?

SQL cannot directly modify the constraints of added columns, but needs to save the country through the following methods: Delete reconstruction: Rebuild after deleting the original constraint, but the risk is huge. When the data is large, it will take time and lead to temporary unavailability; New column migration: Create new columns and copy data, and then delete old columns, which is relatively safe but troublesome; use views: Create views to apply new constraints, and will not modify the underlying table, but only take effect at the view level.

Apr 09, 2025 pm 01:12 PM
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How to delete added columns in SQL?

How to delete added columns in SQL?

SQL cannot directly delete added columns, and an alternative method is required. You can choose to modify the table structure to create a new table, or modify the column name to make it no longer use. The former is more thorough and the latter is more efficient. No matter which method is used, you should pay attention to backup, testing, transaction processing and monitoring to ensure data security and successful operation.

Apr 09, 2025 pm 01:09 PM
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How to add self-increment columns in existing tables in SQL?

How to add self-increment columns in existing tables in SQL?

In MySQL, to add self-increase columns to existing tables, you need to go through steps: add new columns, set as self-increase attributes, not set as primary keys; use auxiliary columns to update existing data, fill in self-increase columns; set new columns as primary keys, and add other constraints.

Apr 09, 2025 pm 01:06 PM
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How to add computed columns in SQL?

How to add computed columns in SQL?

How to add computed columns in SQL? Temporary calculation: Using the SELECT statement, there is no need to modify the table structure, and the calculation results only exist in the query results. Permanent save: Add new columns to the table, use the UPDATE statement to fill the data, and the calculation results are permanently saved in the table, but are not automatically updated. Usage views: Create virtual tables, encapsulate calculation results, easy to use, and does not occupy storage space.

Apr 09, 2025 pm 01:03 PM
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How to add columns in SQLite?

How to add columns in SQLite?

SQLite adds columns using the ALTER TABLE statement, but pay attention to the following potential problems and best practices: the default value is NULL and needs to be updated manually; select the appropriate data type to improve efficiency; concurrent access requires consideration of locks or transactions; adding columns in large tables may take time, consider optimization strategies; update data after adding columns in step-by-step operations, and add constraints if necessary; avoid frequent column additions, use indexes, and regularly maintain databases to improve performance.

Apr 09, 2025 pm 01:00 PM
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What should I pay attention to when adding columns?

What should I pay attention to when adding columns?

Adding columns involves differentiated processing of different database systems, and the database to be used needs to be clearly used. Adding columns means modifying the table structure, affecting performance, and you need to operate with caution and follow best practices. When it comes to locking table operations, you need to choose the right time. Advanced usage includes setting default values, constraints, and indexes. When debugging error messages, performance optimization needs to consider index impact. Programming habits focus on specifications and comments.

Apr 09, 2025 pm 12:57 PM
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How to add columns in SQL graphical tools?

How to add columns in SQL graphical tools?

Add Columns in SQL Graphics Tool: Select the table to which columns you want to add. Right-click and select "Alter Table" or similar options. Defines the properties of the new column (name, data type, length, or whether it is empty). Specifies the default value for the new column, if applicable. Select the appropriate data type to avoid data errors. Use meaningful column names. Consider the performance impact when performing column addition operations on large tables. Always back up the database before the operation to prevent data loss.

Apr 09, 2025 pm 12:54 PM
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