SQL’s core logic: relational databases store data in two-dimensional tables (rows = records, columns = fields); SQL statements query (SELECT), filter (WHERE), aggregate (GROUP BY), join (JOIN), and sort (ORDER BY) data in these tables. Whether MySQL, PostgreSQL, SQLite, or BigQuery, core SQL syntax is largely universal — learn one and you largely know them all.
## Core SQL Syntax: From Basics to Practical Use
**Basic query**:
“`sql
SELECT column1, column2
FROM table_name
WHERE condition
ORDER BY column1 DESC
LIMIT 100;
“`
**Aggregate query** (calculating statistics):
“`sql
SELECT city,
COUNT(*) AS order_count,
SUM(amount) AS total_revenue,
AVG(amount) AS avg_order_value
FROM orders
WHERE order_date >= ‘2024-01-01’
GROUP BY city
HAVING COUNT(*) > 100
ORDER BY total_revenue DESC;
“`
**Multi-table JOIN**:
“`sql
SELECT u.name, u.city, COUNT(o.id) AS order_count
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
GROUP BY u.id, u.name, u.city;
“`
JOIN is both the most important and most error-prone part of SQL: INNER JOIN only keeps rows with matches in both tables; LEFT JOIN keeps all rows from the left table (rows without right-table matches show NULL). Understanding JOIN’s set logic is key to improving SQL capability.
## Practical Workplace SQL Scenarios
**Operations**: counting user registration volumes and next-day retention rates by channel; **Product**: analyzing user conversion funnel drop-off at each step from registration to first payment; **Finance**: summarizing monthly revenue by region and product line compared against budget; **Marketing**: analyzing customer acquisition cost (CAC) and ROI across different advertising channels. 90% of workplace SQL needs can be met with SELECT + WHERE + GROUP BY + JOIN.
## Recommended Learning Resources
Free resources: SQLZoo (interactive practice), Mode Analytics SQL Tutorial, LeetCode Database problems (interview-focused). Tools: DBeaver or TablePlus (database clients); no local database needed — practice with online SQL sandboxes (e.g., sqlfiddle.com). Practicing with real business data is more efficient than doing exercises.
See [Workplace Data Literacy](https://sunqi.org/data-literacy-workplace-en/) and [Mode Analytics SQL Tutorial](https://mode.com/sql-tutorial/).
