CASE函数经常看到，COALESCE不怎么用到，但好像在别人的代码中看到过。今天学习一下。
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COALESCE Syntax PurposeCOALESCE returns the first non-null expr in the expression list. You must specify at least two expressions. If all occurrences of expr evaluate to null, then the function returns null. Oracle Database uses short-circuit evaluation. The database evaluates each expr value and determines whether it is NULL, rather than evaluating all of the expr values before determining whether any of them is NULL. If all occurrences of expr are numeric data type or any nonnumeric data type that can be implicitly converted to a numeric data type, then Oracle Database determines the argument with the highest numeric precedence, implicitly converts the remaining arguments to that data type, and returns that data type. This function is a generalization of the NVL function. You can also use COALESCE as a variety of the CASE expression. For example, COALESCE(expr1, expr2) is equivalent to: CASE WHEN expr1 IS NOT NULL THEN expr1 ELSE expr2 END Similarly, COALESCE(expr1, expr2, ..., exprn) where n >= 3, is equivalent to: CASE WHEN expr1 IS NOT NULL THEN expr1 ELSE COALESCE (expr2, ..., exprn) END Examples The following example uses the sample oe.product_information table to organize a clearance sale of products. It gives a 10% discount to all products with a list price. If there is no list price, then the sale price is the minimum price. If there is no minimum price, then the sale price is "5": SELECT product_id, list_price, min_price, COALESCE(0.9*list_price, min_price, 5) "Sale" FROM product_information WHERE supplier_id = 102050 ORDER BY product_id; See Also: Table 3–10, " Implicit Type Conversion Matrix" on page 3-40 for more information on implicit conversion and "Numeric Precedence" on page 3-14 for information on numeric precedence See Also: NVL on page 5-173 and "CASE Expressions" on page 6-5 COALESCE
PRODUCT_ID LIST_PRICE MIN_PRICE Sale ---------- ---------- ---------- ---------- 1769 48 43.2 1770 73 73 2378 305 247 274.5 2382 850 731 765 3355 5
