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Core Oracle SQL
DUAL table FETCH FIRST ROWNUM NVL NVL2 DECODE CASE expression TO_CHAR TO_DATE TO_NUMBER
Queries & Joins
INNER JOIN LEFT OUTER JOIN RIGHT OUTER JOIN FULL OUTER JOIN CROSS JOIN Self join Subquery Correlated subquery EXISTS Scalar subquery
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GROUP BY HAVING ROLLUP CUBE GROUPING SETS LISTAGG PIVOT UNPIVOT UNION MINUS
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INSERT ALL MERGE RETURNING INTO COMMIT ROLLBACK SAVEPOINT SET TRANSACTION SELECT FOR UPDATE Flashback query AS OF Error logging LOG ERRORS
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JSON_OBJECT JSON_ARRAY JSON_VALUE JSON_QUERY JSON_TABLE IS JSON constraint XMLTYPE XMLQUERY XMLTABLE JSON_TRANSFORM
Performance & Security
EXPLAIN PLAN DBMS_XPLAN display Optimizer hint Index skip scan concept Bitmap index Partition pruning Virtual private database concept Fine-grained auditing concept Roles and privileges Row limiting pagination
Oracle SQL Complete Course / Queries & Joins

Scalar subquery

Learn Scalar subquery in Oracle SQL through the exact construct, a dedicated example, a boundary case, and a mastery check.

10 min read • Beginner

Scalar subquery is an atomic Oracle SQL lesson inside Queries & Joins. The example uses the specific syntax, command, API, instruction, or language rule named by this lesson.

Study the construct itself, then change one relevant input and predict the result before running it. That keeps this lesson focused on Scalar subquery rather than on generic Oracle SQL syntax.

Topic-specific example

oraclesql-02-10.sql
sql Try It Live 
SELECT id,(SELECT MAX(created_at) FROM orders o WHERE o.user_id=u.id) last_order FROM users u;
Edge case / engineering note

Test Scalar subquery with an empty result, NULL/missing value, duplicate key, or concurrent operation where relevant; database behavior should be verified at the boundary shown by this example.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

Scalar subquery Mastery Check →
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EXISTS
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CONNECT BY