未终止的美元报价

Question

PostgreSql（11版）如何使用IF语句？我只尝试了原始 IF 用法，但遇到了问题（“IF”处或附近的语法错误）。为了解决这个问题，人们建议使用“do &&”，但它不起作用（未终止的美元报价从 SQL DO $$ BEGIN IF 中的位置 3 开始）。这是我的 SQL 代码：

DO $$
BEGIN
  IF (NOT EXISTS (SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'categories')) THEN
    CREATE TABLE IF NOT EXISTS categories
    (
      id   SERIAL NOT NULL,
      name character varying(40),
      CONSTRAINT categories_pkey PRIMARY KEY (id)
    );

    INSERT INTO categories (name) VALUES ('Games');
    INSERT INTO categories (name) VALUES ('Multimedia');
    INSERT INTO categories (name) VALUES ('Productivity');
    INSERT INTO categories (name) VALUES ('Tools');
    INSERT INTO categories (name) VALUES ('Health');
    INSERT INTO categories (name) VALUES ('Lifestyle');
    INSERT INTO categories (name) VALUES ('Other');
  END IF;
END
$$;

如果表不存在，我只需要创建表并插入一些初始化数据。

Answer 1

有些平台不支持美元报价。在您的具体示例中，您应该在最后一个END. 您可能还需要添加一条DECLARE语句。

DO
$$
DECLARE
BEGIN
    IF (NOT EXISTS (SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'categories')) 
    THEN
        CREATE TABLE IF NOT EXISTS categories
        (
          id   SERIAL NOT NULL,
          name character varying(40),
          CONSTRAINT categories_pkey PRIMARY KEY (id)
        );

        INSERT INTO categories (name) VALUES ('Games');
        INSERT INTO categories (name) VALUES ('Multimedia');
        INSERT INTO categories (name) VALUES ('Productivity');
        INSERT INTO categories (name) VALUES ('Tools');
        INSERT INTO categories (name) VALUES ('Health');
        INSERT INTO categories (name) VALUES ('Lifestyle');
        INSERT INTO categories (name) VALUES ('Other');
    END IF;
END;
$$  LANGUAGE PLPGSQL;

对于无法识别美元报价的平台，您可以'改用。不过，您需要对'匿名函数体内的任何内容进行转义。

像这样：

DO
'
DECLARE
BEGIN
    IF (NOT EXISTS (SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = ''categories'')) 
    THEN
        CREATE TABLE IF NOT EXISTS categories
        (
          id   SERIAL NOT NULL,
          name character varying(40),
          CONSTRAINT categories_pkey PRIMARY KEY (id)
        );

        INSERT INTO categories (name) VALUES (''Games'');
        INSERT INTO categories (name) VALUES (''Multimedia'');
        INSERT INTO categories (name) VALUES (''Productivity'');
        INSERT INTO categories (name) VALUES (''Tools'');
        INSERT INTO categories (name) VALUES (''Health'');
        INSERT INTO categories (name) VALUES (''Lifestyle'');
        INSERT INTO categories (name) VALUES (''Other'');
    END IF;
END;
'  LANGUAGE PLPGSQL;

DBFiddle查看工作示例。