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.

Know first

Database

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

idnamecity

1XiaoliShenzhen

2AjiHangzhou

3Xiao LinShanghai

Quick checkChoose the best answer

For a current user’s incomplete tasks, which SQL boundary is safer?

Filter by current user and incomplete status, returning only fields the page needsQuery every task and hide other users’ rows in the browserUse SELECT * as long as incomplete status is filtered

You can say this to an AI Agent

Make the task-list endpoint query only the current user’s incomplete tasks and return only fields used by the page. Test with two users’ data: each must see only their incomplete items, and the response must omit unnecessary sensitive columns.

When to use it

Filter and sort records
Join related tables
Aggregate totals and counts
Update data inside a transaction

When NOT to use it

Concatenate untrusted input into a query
Select every column when only a few are needed
Run an unbounded update or delete
Assume a database permission replaces application authorization

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