在HSQLDB

Question

在加入操作期間由另一個值替換NULL

當運行使用HSQLDB 2.2.5，的results.csv內容下面的腳本（q.sql）成爲以下：

results.csv：

C1,C2 
[null],e 
a,b 

q.sql：

DROP TABLE "PUBLIC"."TABLE1" IF EXISTS; 

CREATE TABLE "PUBLIC"."TABLE1" (
    "C1" VARCHAR(10), 
    "C2" VARCHAR(10), 
    "C3" VARCHAR(10) 
); 

INSERT INTO "PUBLIC"."TABLE1" ("C1", "C2", "C3") VALUES ('a', 'b', 'c'); 
INSERT INTO "PUBLIC"."TABLE1" ("C1", "C2", "C3") VALUES ('d', 'e', 'f'); 

DROP TABLE "PUBLIC"."TABLE2" IF EXISTS; 

CREATE TABLE "PUBLIC"."TABLE2" (
    "C1" VARCHAR(10), 
    "C2" VARCHAR(10), 
); 

INSERT INTO "PUBLIC"."TABLE2" ("C1", "C2") VALUES ('a', 'b'); 
INSERT INTO "PUBLIC"."TABLE2" ("C1", "C2") VALUES ('u', 'v'); 

DROP TABLE "PUBLIC"."TABLE3" IF EXISTS; 

CREATE TABLE "PUBLIC"."TABLE3" (
    "C1" VARCHAR(10), 
    "C2" VARCHAR(10) 
); 

INSERT INTO "PUBLIC"."TABLE3" (
    "C1", 
    "C2" 
) 
SELECT 
(SELECT 
"T2"."C1" AS "T2C1" 
FROM "PUBLIC"."TABLE2" "T2" 
WHERE "T2"."C1" = "T1"."C1") AS "T2C1", 
"T1"."C2" AS "T1C2" 
FROM "PUBLIC"."TABLE1" "T1" 
ORDER BY "T2C1", "T1C2"; 

* *DSV_COL_DELIM=, 
* *DSV_ROW_DELIM=\n 
* *DSV_TARGET_FILE=results.csv 

\x "PUBLIC"."TABLE3" 

哪有我改變我的腳本，使得「[null]」被替換爲「？」在生成的CSV文件中？

Answer 1

不是插入表中的值，而是使用case語法插入來檢查值是否爲空。

事情是這樣的：

case when value = [null] then ? else variable end 

Answer 2

取決於用到dbengine：

爲HSQL ...任何3個工作的依據：hSQLDB.org

COALESCE（MOST）

NVL （oracle）

ifNull（mysql） 編輯添加（...（哎呀，我忘了！）

SELECT 
coalesce((SELECT 
"T2"."C1" AS "T2C1" 
FROM "PUBLIC"."TABLE2" "T2" 
WHERE "T2"."C1" = "T1"."C1"),'?') AS "T2C1", 
"T1"."C2" AS "T1C2" 
FROM "PUBLIC"."TABLE1" "T1" 
ORDER BY "T2C1", "T1C2";