SELECT first_name || ' ' || last_name AS full_name

FROM employees;

SELECT

UPPER(email) AS email_upper,

LOWER(department) AS dept_lower

FROM employees;

SELECT

SUBSTRING(phone, 1, 3) AS area_code,

LENGTH(description) AS desc_length

FROM products;

SELECT * FROM customers

WHERE email LIKE '%@gmail.com';

Concatenation: || (PostgreSQL/Oracle), CONCAT() (MySQL), + (SQL Server)

LIKE patterns: % (any characters), _ (one character)

String functions are case-sensitive by default

NULL concatenation returns NULL (use COALESCE)

Position/Index 1-based, not 0-based

Core references in this topic include CONCAT, SUBSTRING, TRIM. Learn what each one does, when to use it, and the execution or engine rules that matter.

CONCAT

Builds a string from multiple values.

CONCAT(first_name, ' ', last_name)

SUBSTRING

Extracts part of a string based on starting position and length.

SUBSTRING(email FROM 1 FOR 5)

TRIM

Removes leading and trailing characters, commonly whitespace.

TRIM(customer_name)

UPPER / LOWER

Normalizes text casing for comparisons and presentation.

UPPER(country_code)

REPLACE

Substitutes one substring with another inside a string value.

REPLACE(phone, '-', '')

LENGTH

Returns the number of characters in a string value.

LENGTH(product_name)

WHERE

Filters rows before projection and sorting. It decides which rows continue through the query pipeline.

SELECT ... FROM table WHERE condition;

Most performance issues start with a weak WHERE clause or a missing supporting index.