Clause
Enhanced Definition
An SQL clause is a distinct, keyword-driven component of an SQL statement that specifies a particular operation, condition, or characteristic. In the context of DB2 for z/OS, clauses define how data is retrieved, manipulated, or defined within relational tables, forming the core logic of database interactions. In the context of SQL (Structured Query Language) on z/OS, a clause is a distinct syntactic component of an SQL statement that specifies a particular action, condition, or attribute. Clauses modify or extend the primary operation of an SQL statement, such as `SELECT`, `INSERT`, `UPDATE`, or `DELETE`, to define precisely how data should be manipulated or retrieved from a DB2 for z/OS database.
Key Characteristics
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- Syntactic Building Block: Each clause begins with a specific keyword (e.g.,
SELECT,FROM,WHERE,GROUP BY,ORDER BY) and contributes to the overall structure and meaning of an SQL statement. - Modifies Statement Behavior: Clauses specify the columns to retrieve, the tables to query, the conditions for filtering rows, the grouping criteria for aggregation, or the order of the result set.
- Logical Processing Order: While written in a specific order, DB2's SQL optimizer processes clauses in a defined logical sequence (e.g.,
FROM,WHERE,GROUP BY,HAVING,SELECT,ORDER BY), which is crucial for understanding query execution. - Versatility Across SQL Types: Clauses are fundamental to Data Manipulation Language (DML) statements like
SELECT,INSERT,UPDATE,DELETE, and also appear in Data Definition Language (DDL) statements (e.g.,CREATE TABLEwithPRIMARY KEYorFOREIGN KEYclauses). - Performance Impact: The choice and construction of clauses, especially
WHEREandJOINclauses, significantly influence the access path chosen by the DB2 optimizer and thus the performance of queries on z/OS.
- Syntactic Building Block: Each clause begins with a specific keyword (e.g.,
Use Cases
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- Filtering Data: The
WHEREclause is used to specify conditions that rows must meet to be included in the result set, such asWHERE CUST_ID = '12345'orWHERE ORDER_DATE BETWEEN '2023-01-01' AND '2023-01-31'. - Aggregating Results:
GROUP BYclauses are used with aggregate functions (e.g.,COUNT,SUM,AVG) to group rows based on common column values, often followed by aHAVINGclause to filter these groups. - Ordering Output: The
ORDER BYclause sorts the rows in the final result set based on one or more columns, either in ascending (ASC) or descending (DESC) order, for presentation or further processing. - Combining Data from Multiple Tables:
JOINclauses (e.g.,INNER JOIN,LEFT JOIN) are essential for retrieving related data stored across different tables in a DB2 database, linking them based on common column values
- Filtering Data: The
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