Reference

SQL String Functions

Measure, reshape, clean, and combine text values.

Text literals use single quotes:'active'. Double quotes mean an identifier (a column or table name), which is a common beginner mix-up.

Common functions

Function	Returns
`length(s)`	Number of characters.
`lower(s)` / `upper(s)`	Change case.
`trim(s)`	Remove leading/trailing spaces.
`substring(s, 1, 3)`	Slice from position, length.
`replace(s, 'a', 'b')`	Replace all occurrences.
`split_part(s, '@', 2)`	Nth piece after splitting on a delimiter.
`concat(a, b)` or `a \|\| b`	Join strings together.

Cleaning and combining

Normalize and join

select first_name || ' ' || last_name        as full_name,
       lower(trim(email))                   as email_clean,
       split_part(email, '@', 2)            as email_domain
from   customers

Case-insensitive matching

Two equivalent ways

-- normalize both sides
where lower(state) = 'pa'

-- or use ILIKE (DuckDB / Postgres)
where state ilike 'pa'

Dialect note

Concatenation differs by engine: || works in DuckDB, Postgres, SQLite, and Oracle. SQL Server uses + (orCONCAT). MySQL needs CONCAT unlessPIPES_AS_CONCAT is enabled. CONCAT(...) is the most portable choice.

Lesson

Why SQL Data Types Matter

See why text, numbers, dates, and booleans behave differently.

Lesson

Numbers and Calculations in SQL

Compare, sort, and calculate with numeric values.

Lesson

Why SQL Strings Need Quotes

Filter text values with string literals.

Lesson

Working with Dates and Times in SQL

Compare and sort date values chronologically.