SQL code examples · runnable

20 runnable SQL code examples.

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SQL

Inner, left, right and full outer joins in SQLINNER JOIN keeps only matching rows; LEFT, RIGHT and FULL OUTER JOIN also keep the rows with no partner and fill the gaps with NULL. One pair of tables, every join, the ON vs WHERE trap, and FULL JOIN in MySQL.How to update one table from another in SQLUpdate rows in one table with values from another: UPDATE … FROM where your database has it, the portable correlated subquery with WHERE EXISTS, several columns at once, and totals from an aggregate.How to add a column with a default value in SQLAdd a column with a default value to an existing table with ALTER TABLE … ADD COLUMN … DEFAULT, see why NOT NULL needs a default, rename and drop columns, change nullability, and rebuild a SQLite table to change a column type.How to find and delete duplicate rows in SQLFind duplicate values in SQL with GROUP BY and HAVING COUNT(*) > 1, list every duplicate row with a window count, then delete all copies but one, keeping the lowest id or the newest row, and add a UNIQUE index so duplicates cannot come back.How to combine rows into one string in SQLCombine values from many rows into one comma-separated string in SQL with string_agg or group_concat: one list per group with GROUP BY, sorted inside the list, without duplicates, with NULLs handled, and the PostgreSQL, MySQL and SQL Server forms.How to get only the date from a datetime in SQLStrip the time from a datetime in SQL: date() in SQLite, CAST(x AS date) elsewhere, filtering one day with a range that keeps the index, grouping by day or month, and getting the time part.How to insert multiple rows in one SQL statementInsert several rows with one INSERT: list them after VALUES as (…), (…), (…), let missing columns take their defaults, get the new ids back with RETURNING, and know what happens when one row fails.How to select the first row in each group in SQLPick one row per group in SQL, such as the latest order per customer, with ROW_NUMBER() OVER (PARTITION BY …) and WHERE rn = 1. Also covers top N per group, the join-to-MAX and correlated subquery forms and the ties they return, RANK() for keeping ties, and the SQLite bare-column shortcut.How to insert rows from a SELECT in SQLCopy rows from one table into another with INSERT INTO … SELECT, mix literals into the copied rows, skip rows that are already there, create and fill a new table with CREATE TABLE … AS, and copy one column into another.How to write IF/THEN in a SQL SELECTWrite IF/THEN logic inside a SQL SELECT with CASE WHEN … THEN … ELSE … END, the form every database accepts. Covers the simple CASE form, OR and IN inside a WHEN, CASE in ORDER BY and aggregates, and the iif(), COALESCE and NULLIF shortcuts.How to delete rows using a join in SQLDelete rows based on a match in another table: the portable DELETE … WHERE EXISTS and WHERE id IN (SELECT … JOIN …) forms, NOT EXISTS for rows with no match, a preview SELECT first, RETURNING, and the DELETE … JOIN and USING syntax of MySQL, SQL Server and PostgreSQL.UNION vs UNION ALL in SQLUNION removes duplicate rows from the combined result, UNION ALL keeps every row and skips the de-duplication step. See both on real data, plus the column rules, ORDER BY, a source label column, and INTERSECT and EXCEPT.How to group by multiple columns in SQLGroup by two or more columns in SQL with GROUP BY region, year: one row per distinct combination, with COUNT, SUM and AVG per group, HAVING versus WHERE, GROUP BY versus DISTINCT, subtotals, and the "must appear in the GROUP BY clause" error.How to escape a single quote in SQLEscape a single quote in SQL by doubling it ('It''s'), in INSERT, WHERE and LIKE. Also covers char(39) and replace(), why double quotes and backslashes don't work, and the PostgreSQL, MySQL and SQL Server alternatives.How to limit rows and paginate in SQLReturn the top N rows with ORDER BY … LIMIT, fetch page k with LIMIT and OFFSET, pick the nth row, keep pages stable with a tie-breaker, page fast with a keyset WHERE, and keep ties with RANK(). Plus TOP and OFFSET … FETCH for SQL Server.How to get the ID of an inserted row in SQLGet the id of the row you just inserted with INSERT … RETURNING id, or last_insert_rowid() in SQLite. Covers multi-row inserts, INTEGER PRIMARY KEY and AUTOINCREMENT, child rows, and the PostgreSQL, MySQL and SQL Server forms.How to insert or update if a row exists in SQL (upsert)Insert a row, or update it if the key already exists, in one statement with INSERT … ON CONFLICT DO UPDATE. Covers counters, insert-if-not-exists with DO NOTHING, why INSERT OR REPLACE is not an upsert, a portable NOT EXISTS form, and the MySQL and SQL Server syntax.How to find rows with no match in another table in SQLFind the rows in one table that have no match in another with NOT EXISTS, a LEFT JOIN anti-join or EXCEPT, and see why one NULL makes NOT IN return nothing.How to write a recursive query for tree data in SQLQuery a parent_id tree with WITH RECURSIVE: every descendant of a node with its depth, a path string and an indented outline, the ancestors of a node, descendant counts, and guards that stop a cycle from looping forever.How to count rows with a condition in SQLCount only the rows that match a condition in SQL with SUM(CASE WHEN … THEN 1 ELSE 0 END) or COUNT(*) FILTER (WHERE …). Covers several counts in one query, counts per group, the SUM(condition) shortcut, COUNT(*) vs COUNT(column) vs COUNT(DISTINCT), and percentages.

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SQL examples, explained

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