Backend

SQL

You might say

How do I write a query for users who placed an order in the last seven days?

Ask a relational database to read or change structured recordsSQL can select, filter, join, insert, update, and delete data in relational databases. Parameterize user values instead of building query strings, and inspect indexes and execution plans when a query becomes slow. A correct query still needs authorization around it.

SELECT name, city FROM users WHERE city = 'Shenzhen'

idnamecity

1XiaoliShenzhen

2AjiHangzhou

3Xiao LinShanghai

Anatomy

SELECT name FROM users WHERE city = ?

idnamecity

1MiaShenzhen

SELECT, INSERT, UPDATE or DELETE, decide whether to read or modify

Relational data collection to be operated; first confirm the table name and field structure

WHERE limits the scope of influence; placeholders such as question marks are safely passed by parameterized queries.

Variants

SELECT

SELECTRead matching rows

Read records without changing them.

INSERT

INSERTAdd a new row

Create a new record.

UPDATE

UPDATEChange the row hit by WHERE

Update selected records; check the WHERE clause.

DELETE

DELETEDelete the row hit by WHERE

Remove selected records after checking recovery.

Typical use cases

Order query

Query usersSELECT to find records that meet the conditions

SELECT id, name, planFROM usersWHERE plan = 'pro';

Result: u_23 · Alex Chen · pro

User lookup

New taskINSERT creates a record

ExecuteINSERT INTO tasks ... → Returnid = task_8842

Analytics total

Update dataUPDATE only modifies the specified user

UPDATE usersSET city = 'Shenzhen'WHERE id = 'u_23';

Affects 1 row · Modification successful

Data update

Delete recordConfirm WHERE before DELETE

SELECT WHERE id=t_42→Confirm 1 line→DELETE

Avoid accidentally deleting the entire table

Quick decision

SQL vs NoSQL: which fits your data?

SQL is a query language; the actual choice is between a relational database and a specific NoSQL database. Compare them by data relationships, access patterns, and consistency needs—not by assuming one is faster.

Decision point	Relational (SQL)	NoSQL database
Data structure	Stores structured records in tables and connects related data through fields.	Uses models such as document, key-value, wide-column, or graph.
Decision point	Often the direct fit when joins, constraints, and transactions matter.	Can fit a well-defined data model and access pattern with a purpose-built product.
Typical fit	Orders, accounts, and other related business records that need consistency.	Key-value caches, documents, event records, or graph relationships.